feat(physics): add differential flat BSP traversal

Port every current containment, overlap, walkable, and six-path moving-collision query to immutable integer-indexed assets behind a graph-authoritative referee. Exact synthetic, installed-DAT, complete-resolver, and zero-allocation gates prove bit-identical behavior before connected dual publication.

Co-authored-by: OpenAI Codex <codex@openai.com>

AGENTS.md

@@ -169,9 +169,16 @@ interface shares the render package's single mmap. Two complete
 29,908,271,024-byte bakes produced 2,232,170 typed keys, zero failures, and the
 same SHA-256 across 16 and 7 workers. Corruption, cancellation, installed
 package, Release build, and 8,371-test / 5-skip gates pass.
+Slice I4 is complete: every current containment, overlap, walkable, and
+six-path moving-collision entry point has an integer-indexed flat shadow port.
+The exact referee passes 13 focused tests, 7,500 installed-DAT comparisons,
+1,200 complete resolver frames, and 10,000 warmed iterations at zero managed
+bytes with no mismatch. Release build and 8,384 solution tests / 5 skips pass.
+Production remains graph-authoritative until the connected shadow and cutover
+gates.
 Slice H's retained UI/frame, bounded-light, and pooled/borrowed/direct network
 work is complete; the exact seven-checkpoint connected lifecycle/reconnect gate
-passes. Slice I is active at I4 from
+passes. Slice I is active at I5 from
 `docs/plans/2026-07-25-modern-runtime-slice-i.md`. The exact G4 visual rollback
 is `git revert ef1d263337997bb030eadb7b8e71d73dc659907a`; do not revert G3 or
 the portal-warmup corrections. Evidence:
@@ -179,7 +186,8 @@ the portal-warmup corrections. Evidence:
 `docs/research/2026-07-25-slice-g5-production-profile.md` and
 `docs/research/2026-07-25-slice-i1-transition-scratch.md` and
 `docs/research/2026-07-25-slice-i2-flat-collision-schema.md` and
-`docs/research/2026-07-25-slice-i3-prepared-collision-package.md`.
+`docs/research/2026-07-25-slice-i3-prepared-collision-package.md` and
+`docs/research/2026-07-25-slice-i4-flat-bsp-differential.md`.
 
 **Structural prerequisite before new M4 subsystem work:** all eight
 behavior-preserving `GameWindow` decomposition slices and the automated

CLAUDE.md

@@ -167,9 +167,16 @@ interface shares the render package's single mmap. Two complete
 29,908,271,024-byte bakes produced 2,232,170 typed keys, zero failures, and the
 same SHA-256 across 16 and 7 workers. Corruption, cancellation, installed
 package, Release build, and 8,371-test / 5-skip gates pass.
+Slice I4 is complete: every current containment, overlap, walkable, and
+six-path moving-collision entry point has an integer-indexed flat shadow port.
+The exact referee passes 13 focused tests, 7,500 installed-DAT comparisons,
+1,200 complete resolver frames, and 10,000 warmed iterations at zero managed
+bytes with no mismatch. Release build and 8,384 solution tests / 5 skips pass.
+Production remains graph-authoritative until the connected shadow and cutover
+gates.
 Slice H's retained UI/frame, bounded-light, and pooled/borrowed/direct network
 work is complete; the exact seven-checkpoint connected lifecycle/reconnect gate
-passes. Slice I is active at I4 from
+passes. Slice I is active at I5 from
 `docs/plans/2026-07-25-modern-runtime-slice-i.md`. The exact G4 visual rollback
 is `git revert ef1d263337997bb030eadb7b8e71d73dc659907a`; do not revert G3 or
 the portal-warmup corrections. Evidence:
@@ -177,7 +184,8 @@ the portal-warmup corrections. Evidence:
 `docs/research/2026-07-25-slice-g5-production-profile.md` and
 `docs/research/2026-07-25-slice-i1-transition-scratch.md` and
 `docs/research/2026-07-25-slice-i2-flat-collision-schema.md` and
-`docs/research/2026-07-25-slice-i3-prepared-collision-package.md`.
+`docs/research/2026-07-25-slice-i3-prepared-collision-package.md` and
+`docs/research/2026-07-25-slice-i4-flat-bsp-differential.md`.
 
 **Structural prerequisite before new M4 subsystem work:** all eight
 behavior-preserving `GameWindow` decomposition slices and the automated

docs/plans/2026-04-11-roadmap.md

@@ -1654,13 +1654,20 @@ port in any phase — no separate listing here.
 > defines deterministic immutable collision records and validates exact
 > source identity over synthetic corruption cases and representative installed
 > DATs; production traversal remains on the graph oracle until the later
-> differential gate.
+> differential gate. Slice I3 appends the complete prepared collision package
+> to the existing single mmap. Slice I4 now ports every current containment,
+> overlap, walkable, and six-path moving-collision entry point to the immutable
+> integer-indexed assets. Its exact referee passes 13 focused tests, 7,500
+> installed-DAT comparisons, 1,200 complete resolver frames, and 10,000 warmed
+> iterations at zero managed bytes with no mismatch. Production remains
+> graph-authoritative while I5 dual publication and the connected shadow gate
+> are active. Release build and 8,384 solution tests / 5 skips pass.
 > The historical exact cutover
 > rollback remains
 > `git revert ef1d263337997bb030eadb7b8e71d73dc659907a`. Slice H is complete:
 > retained UI/frame work, exact bounded light selection, and
 > pooled/borrowed/direct network I/O pass 8,292 Release tests / 5 skips and the
-> connected lifecycle/reconnect gate. Slice I is active at I4 from
+> connected lifecycle/reconnect gate. Slice I is active at I5 from
 > [its detailed plan](2026-07-25-modern-runtime-slice-i.md).
 
 **Spec:** `docs/superpowers/specs/2026-07-05-modern-pipeline-design.md` (the
@@ -1685,11 +1692,11 @@ hitch and follows.
 | MP1a | `AcDream.Content` extraction (GL-free MeshExtractor + boundary records out of App) | ✅ SHIPPED 2026-07-05 — user-gated (renders identical, zero tripwires, perf-neutral); 8 commits `651d041e`..`b0758d77` |
 | MP1b | Pak format + `acdream-bake` CLI + mmap `PakReader` + equivalence/full-scale gate | ✅ **SHIPPED 2026-07-24** — 729,888 EnvCell keys → 17,117 unique geometries + 712,771 aliases (42.6×), 751,141 total keys / 38,370 physical blobs, zero failures, 28,192.4 MiB, 81.4 s validated atomic publish; full-DAT gate also corrected a real collision in WB's legacy geometry hash. [Report](../research/2026-07-24-slice-b-full-bake-report.md) |
 | **MP-Alloc (safe batch)** | Reuse per-frame buffers: anim pose, particle draw-list, interior partition, animatedIds/drawableCells sets | ✅ SHIPPED + USER-GATED 2026-07-05 — dense-town frame-time spikes 20–87ms → 6–10ms, alloc spikes (30–75MB single-frame) eliminated, gen2 GC 5–11/window → ~0; user confirms FPS steady. 4 commits `b8c05e2b`..`91afea24` |
-| MP-Alloc (hard sites) | EnvCell settled-camera rebuild gate + physics `Transition` reuse | 🟡 Physics half SHIPPED 2026-07-25 — retail-shaped reset-complete transition/query scratch is bit-identical and 0 B/resolve across five profiles; immutable flat collision storage and its complete deterministic prepared package are shipped; traversal differential work continues in Slice I4. |
+| MP-Alloc (hard sites) | EnvCell settled-camera rebuild gate + physics `Transition` reuse | 🟡 Physics half SHIPPED 2026-07-25 — retail-shaped reset-complete transition/query scratch is bit-identical and 0 B/resolve across five profiles; immutable flat collision storage, its deterministic prepared package, and the zero-mismatch/zero-allocation flat traversal referee are shipped; connected dual publication continues in Slice I5. |
 | MP1c | Streaming cutover to pak + hitch gate | ✅ **SHIPPED 2026-07-24** — typed package-only production source, explicit Setup probes, non-allocating Portal-first lookup, exact translucency metadata, and ordered teardown; capped/uncapped/dense physical routes + user visual gate pass. Uncapped CPU p99 7.825→6.496 ms, GPU p99 3.406→2.706 ms, portal frame max 202.9→39.9 MiB, process allocation −41.8%, zero invalid Setup probes. [Report](../research/2026-07-24-slice-c-prepared-asset-cutover-report.md) |
 | MP2 | Retail particle distance/cell-view degradation: DAT-authored range, exact infinite/finite off-view update branches, emitter-first render scan, user-requested 2× default range | ✅ SHIPPED 2026-07-17 — Aerlinthe hotspot `0x32000223` resolves to retail 64m and defaults to 128m; landscape/entity streaming distance unchanged |
 | MP3 | Arch ECS render world + delta submission (the 300-FPS lever) | ⚪ — note: does NOT fix the steady-state GC churn (that's MP-Alloc); MP3 is the draw-submission throughput lever |
-| MP4 | Zero-alloc frame loop + flat physics data (residual, post-MP-Alloc) | 🟡 ACTIVE — Slice I1 zero-allocation transition scratch, I2 immutable schema, and I3 complete deterministic collision package shipped; I4 flat traversal shadow implementation is active. |
+| MP4 | Zero-alloc frame loop + flat physics data (residual, post-MP-Alloc) | 🟡 ACTIVE — Slice I1 zero-allocation transition scratch, I2 immutable schema, I3 deterministic collision package, and I4 exact flat traversal referee shipped; I5 dual publication and connected shadow verification are active. |
 | MP5 | Job-system parallelism | ⚪ stretch, evidence-gated |
 
 ---

docs/plans/2026-05-12-milestones.md

@@ -128,8 +128,14 @@ prepared-collision interface over the existing single mmap. Two complete
 29,908,271,024-byte bakes contain 2,232,170 keys with zero failures and the
 same SHA-256 across different worker counts; cancellation/corruption gates and
 8,371 solution tests / 5 skips pass.
+Slice I4 ports every current collision entry point to integer-indexed flat
+assets behind a graph-authoritative referee. Thirteen focused tests, 10,000
+randomized static/swept comparisons, 5,000 walkable comparisons, 7,500
+installed-DAT comparisons, and 1,200 complete resolver frames have exact
+state/float-bit equality; 10,000 warmed flat iterations allocate zero managed
+bytes. Release build and 8,384 solution tests / 5 skips pass.
 Slice H is complete: retained UI/frame work, exact bounded light selection,
-and pooled/borrowed/direct network I/O pass. Slice I is active at I4 under
+and pooled/borrowed/direct network I/O pass. Slice I is active at I5 under
 [`2026-07-25-modern-runtime-slice-i.md`](2026-07-25-modern-runtime-slice-i.md).
 The historical exact G4 visual rollback remains
 `git revert ef1d263337997bb030eadb7b8e71d73dc659907a`.

docs/plans/2026-07-25-modern-runtime-slice-i.md

@@ -1,8 +1,8 @@
 # Modern runtime Slice I — flat collision assets and zero-allocation physics
 
-**Status:** I0–I3 COMPLETE — retail oracle, zero-allocation transition
+**Status:** I0-I4 COMPLETE - retail oracle, zero-allocation transition
-scratch, deterministic immutable collision records, and complete prepared
+scratch, deterministic immutable collision records, complete prepared
-package fixed; I4 active
+package, and exact flat-traversal differential gate fixed; I5 active
 **Parent:** `2026-07-24-modern-runtime-architecture.md`, Slice I
 **Purpose:** remove parsed DAT object graphs and steady collision allocations
 without changing one retail collision decision, float, comparison, or traversal
@@ -213,6 +213,16 @@ tests / 5 skips pass. Production traversal remains graph-only. Evidence:
 Gate: zero differential mismatch. Any mismatch blocks cutover; no tolerance
 band is permitted for deterministic scalar results.
 
+**PASSED 2026-07-25:** the integer-indexed shadow ports every current
+containment, overlap, walkable, and six-path moving-collision entry point.
+The 13-test referee completed 10,000 randomized static/swept comparisons,
+5,000 walkable comparisons, 7,500 installed-DAT comparisons across three
+representative cells, all six dispatcher paths, and 1,200 complete resolver
+frames with exact state and float-bit equality. Ten thousand warmed flat
+iterations allocated zero managed bytes. Production remains graph-authoritative
+until I5/I6. Release build and 8,384 solution tests / 5 skips pass. Evidence:
+[`../research/2026-07-25-slice-i4-flat-bsp-differential.md`](../research/2026-07-25-slice-i4-flat-bsp-differential.md).
+
 ### I5 — dual publication and connected shadow gate
 
 1. Extend collision preparation so a `LandblockBuild` carries immutable flat

docs/research/2026-07-25-slice-i4-flat-bsp-differential.md

@@ -0,0 +1,87 @@
+# Slice I4 - differential flat BSP traversal
+
+**Date:** 2026-07-25
+
+**Scope:** Modern Runtime Slice I4 only
+
+**Behavioral boundary:** the new integer-indexed traversal is a shadow
+implementation. The parsed-DAT graph remains production-authoritative until
+the Slice I5 connected shadow gate and Slice I6 cutover.
+
+## Result
+
+`FlatBspQuery` now ports every collision entry point used by the current
+runtime over immutable, integer-indexed nodes and polygon ranges:
+
+- point-in-cell;
+- sphere/cell overlap;
+- static sphere/polygon overlap;
+- swept sphere/polygon time-of-impact;
+- walkable polygon selection;
+- solid and solid-polygon traversal;
+- the complete six-path moving-collision dispatcher.
+
+The port preserves retail's traversal order, strict comparisons, early
+returns, and mutable query state. The positive child remains first where
+retail visits it first; the negative child observes every valid-sphere,
+contact, path, slide, and candidate mutation made by the positive child.
+Only `BSPQuery.PolygonHitsSpherePrecise`, which is independent of graph
+storage, is shared with the existing graph oracle.
+
+`CollisionTraversal` is the narrow temporary referee seam. Production starts
+in `Graph` mode. Tests can run a cloned complete resolver in `Flat` mode, and
+a missing flat asset fails explicitly instead of falling back to the graph.
+Slice I6 removes this representation switch after connected acceptance.
+
+## Retail oracle
+
+The implementation follows the consolidated pseudocode and boundary table in
+[`2026-07-25-slice-i-collision-layout-oracle.md`](2026-07-25-slice-i-collision-layout-oracle.md),
+including:
+
+- `BSPTREE::find_collisions` at `0x0053A440`;
+- `BSPNODE::point_inside_cell_bsp` at `0x0053C1F0`;
+- `BSPNODE::sphere_intersects_cell_bsp` at `0x0053C260`;
+- `BSPNODE::sphere_intersects_poly` at `0x0053CA30`;
+- `BSPNODE::sphere_intersects_solid` at `0x0053CAF0`;
+- `BSPNODE::find_walkable` at `0x0053CC80`;
+- `BSPNODE::sphere_intersects_solid_poly` at `0x0053CD50`;
+- `BSPLEAF::find_walkable` at `0x0053D6F0`.
+
+The same oracle records the cross-checks against ACE master
+`65f092...` and Chorizite DatReaderWriter `c535987...`. Retail remains the
+authority where those references differ.
+
+## Differential evidence
+
+The referee compares scalar results and all stateful outputs by exact float
+bits. It covers selected polygons, vectors, planes, path state, collision and
+object flags, contact-plane writes, final `ResolveResult`, persistent body
+state, and full cell membership.
+
+| Gate | Executions | Result |
+|---|---:|---:|
+| Null, on-plane, radius-equality, equal-candidate, multi-polygon, and deep-tree fixtures | 4 focused fixtures | 0 mismatches |
+| Static and swept overlap randomized sweep | 10,000 paired cases | 0 mismatches |
+| Walkable randomized sweep | 5,000 paired cases | 0 mismatches |
+| Six dispatcher paths | 6 complete transition fixtures | 0 mismatches |
+| Installed-DAT sweep across `0x8A02016E`, `0x8A02017A`, and `0xA9B4013F` | 7,500 randomized cases | 0 mismatches |
+| Complete graph/flat resolver sequences | 1,200 frames | 0 mismatches |
+| Warmed flat traversal | 10,000 iterations | 0 managed bytes |
+
+The installed-DAT gate randomizes points, spheres, movement, insert paths,
+orientations, scale, and boundary values. No tolerance band is used.
+
+## Verification
+
+- focused differential tests: 13 passed;
+- complete Core suite: 3,276 passed / 2 skipped;
+- `git diff --check`: no whitespace errors;
+- full Release build: passed;
+- complete Release suite: 8,384 passed / 5 skipped.
+
+No divergence-register row changes are required. This slice changes the
+storage representation exercised by the referee, not production gameplay
+behavior. Slice I5 next publishes the prepared flat assets alongside the
+current graph under one landblock generation and runs the connected shadow
+gate.

src/AcDream.Core/Physics/BSPQuery.cs

@@ -96,7 +96,7 @@ public static class BSPQuery
     ///
     /// <para>ACE: Polygon.cs polygon_hits_sphere_precise.</para>
     /// </summary>
-    private static bool PolygonHitsSpherePrecise(
+    internal static bool PolygonHitsSpherePrecise(
         in Plane polyPlane,
         ReadOnlySpan<Vector3> verts,
         Vector3 sphereCenter,
@@ -2339,6 +2339,31 @@ public static class BSPQuery
                                                   ref hitPolyId, ref hitNormal);
     }
 
+    /// <summary>
+    /// Allocation-free graph-oracle seam used by the Slice I flat traversal
+    /// differential harness.
+    /// </summary>
+    internal static bool SphereIntersectsPoly(
+        PhysicsBSPNode? node,
+        Dictionary<ushort, ResolvedPolygon> resolved,
+        Vector3 sphereCenter,
+        float sphereRadius,
+        out ushort hitPolyId,
+        out Vector3 hitNormal)
+    {
+        hitPolyId = 0;
+        hitNormal = Vector3.Zero;
+        if (node is null) return false;
+
+        return SphereIntersectsPolyStaticRecurse(
+            node,
+            resolved,
+            sphereCenter,
+            sphereRadius,
+            ref hitPolyId,
+            ref hitNormal);
+    }
+
     private static bool SphereIntersectsPolyStaticRecurse(
         PhysicsBSPNode? node,
         Dictionary<ushort, ResolvedPolygon> resolved,
@@ -2431,6 +2456,38 @@ public static class BSPQuery
         return hitTime < float.MaxValue;
     }
 
+    /// <summary>
+    /// Allocation-free graph-oracle seam used by the Slice I flat traversal
+    /// differential harness.
+    /// </summary>
+    internal static bool SphereIntersectsPolyWithTime(
+        PhysicsBSPNode? node,
+        Dictionary<ushort, ResolvedPolygon> resolved,
+        Vector3 sphereCenter,
+        float sphereRadius,
+        Vector3 movement,
+        out ushort hitPolyId,
+        out Vector3 hitNormal,
+        out float hitTime)
+    {
+        hitPolyId = 0;
+        hitNormal = Vector3.Zero;
+        hitTime = float.MaxValue;
+        if (node is null) return false;
+
+        SphereIntersectsPolyWithTimeRecurse(
+            node,
+            resolved,
+            sphereCenter,
+            sphereRadius,
+            movement,
+            ref hitPolyId,
+            ref hitNormal,
+            ref hitTime);
+
+        return hitTime < float.MaxValue;
+    }
+
     private static void SphereIntersectsPolyWithTimeRecurse(
         PhysicsBSPNode? node,
         Dictionary<ushort, ResolvedPolygon> resolved,

src/AcDream.Core/Physics/CellTransit.cs

@@ -159,15 +159,19 @@ public static class CellTransit
             // neighbour cell whether the sphere intersects its CellBSP.
             // The portal-plane side test is only the unloaded-cell load hint.
             var otherCell = cache.GetCellStruct(otherId);
-            if (otherCell?.CellBSP?.Root is not null)
+            if (otherCell is not null &&
+                CollisionTraversal.HasCellContainment(cache, otherCell))
             {
                 for (int i = 0; i < sphereCount; i++)
                 {
                     var sphere = worldSpheres[i];
                     var otherLocalCenter = Vector3.Transform(
                         sphere.Origin, otherCell.InverseWorldTransform);
-                    bool hit = BSPQuery.SphereIntersectsCellBsp(
+                    bool hit = CollisionTraversal.SphereIntersectsCell(
-                        otherCell.CellBSP.Root, otherLocalCenter, sphere.Radius);
+                        cache,
+                        otherCell,
+                        otherLocalCenter,
+                        sphere.Radius);
                     if (hit)
                     {
                         candidates.Add(otherId);
@@ -372,7 +376,8 @@ public static class CellTransit
                 continue;
 
             var otherCell = cache.GetCellStruct(portal.OtherCellId);
-            if (otherCell?.CellBSP?.Root is null)
+            if (otherCell is null ||
+                !CollisionTraversal.HasCellContainment(cache, otherCell))
             {
                 if (PhysicsDiagnostics.ProbeIndoorBspEnabled)
                 {
@@ -398,8 +403,11 @@ public static class CellTransit
             {
                 var sphere = worldSpheres[i];
                 var localCenter = Vector3.Transform(sphere.Origin, otherCell.InverseWorldTransform);
-                inside = BSPQuery.SphereIntersectsCellBsp(
+                inside = CollisionTraversal.SphereIntersectsCell(
-                    otherCell.CellBSP.Root, localCenter, sphere.Radius);
+                    cache,
+                    otherCell,
+                    localCenter,
+                    sphere.Radius);
 
                 if (PhysicsDiagnostics.ProbeIndoorBspEnabled)
                 {
@@ -599,19 +607,25 @@ public static class CellTransit
         if (start is null) return 0u;
 
         // this->point_in_cell(point) → return this  (:311402-311405)
-        if (PointInCell(start, worldPoint)) return startCellId;
+        if (PointInCell(cache, start, worldPoint)) return startCellId;
 
         if (useStabList)
         {
             // arg3 != 0 → iterate stab_list, GetVisible + point_in_cell (:311444-311465)
             foreach (uint id in start.VisibleCellIds)
-                if (PointInCell(cache.GetCellStruct(id), worldPoint)) return id;
+                if (PointInCell(cache, cache.GetCellStruct(id), worldPoint)) return id;
         }
         else
         {
             // arg3 == 0 → iterate direct portals, GetOtherCell + point_in_cell (:311411-311434)
             foreach (var portal in start.Portals)
-                if (PointInCell(cache.GetCellStruct(portal.OtherCellId), worldPoint)) return portal.OtherCellId;
+                if (PointInCell(
+                        cache,
+                        cache.GetCellStruct(portal.OtherCellId),
+                        worldPoint))
+                {
+                    return portal.OtherCellId;
+                }
         }
 
         return 0u;
@@ -623,11 +637,19 @@ public static class CellTransit
     /// A cell with no hydrated <see cref="CellPhysics.CellBSP"/> returns false (see
     /// <see cref="FindVisibleChildCell"/>'s adaptation note).
     /// </summary>
-    private static bool PointInCell(CellPhysics? cell, Vector3 worldPoint)
+    private static bool PointInCell(
+        PhysicsDataCache cache,
+        CellPhysics? cell,
+        Vector3 worldPoint)
     {
-        if (cell?.CellBSP?.Root is null) return false;
+        if (cell is null ||
+            !CollisionTraversal.HasCellContainment(cache, cell))
+        {
+            return false;
+        }
+
         var local = Vector3.Transform(worldPoint, cell.InverseWorldTransform);
-        return BSPQuery.PointInsideCellBsp(cell.CellBSP.Root, local);
+        return CollisionTraversal.PointInsideCell(cache, cell, local);
     }
 
     /// <summary>
@@ -901,9 +923,14 @@ public static class CellTransit
             {
                 // Interior candidate — point_in_cell via the cell BSP (vtable[0x84]).
                 var cand = cache.GetCellStruct(candId);
-                if (cand?.CellBSP?.Root is null) continue;
+                if (cand is null ||
+                    !CollisionTraversal.HasCellContainment(cache, cand))
+                {
+                    continue;
+                }
+
                 var local = Vector3.Transform(worldSphereCenter, cand.InverseWorldTransform);
-                if (BSPQuery.PointInsideCellBsp(cand.CellBSP.Root, local))
+                if (CollisionTraversal.PointInsideCell(cache, cand, local))
                     return candId;                       // interior-wins, stop (pseudo_c:308819)
             }
             else if (outdoorResult == 0u && containingOutdoorId != 0u && outdoorPickAllowed)
@@ -950,10 +977,15 @@ public static class CellTransit
         if (currentLow >= 0x0100u)
         {
             var cur = cache.GetCellStruct(currentCellId);
-            if (cur?.CellBSP?.Root is not null)
+            if (cur is not null &&
+                CollisionTraversal.HasCellContainment(cache, cur))
             {
                 var curLocal = Vector3.Transform(worldSphereCenter, cur.InverseWorldTransform);
-                if (!BSPQuery.SphereIntersectsCellBsp(cur.CellBSP.Root, curLocal, sphereRadius))
+                if (!CollisionTraversal.SphereIntersectsCell(
+                        cache,
+                        cur,
+                        curLocal,
+                        sphereRadius))
                 {
                     uint recovered = FindVisibleChildCell(
                         cache, currentCellId, worldSphereCenter, useStabList: true);

src/AcDream.Core/Physics/CollisionTraversal.cs

@@ -0,0 +1,226 @@
+using System.Numerics;
+
+namespace AcDream.Core.Physics;
+
+/// <summary>
+/// Temporary Slice I referee mode. Graph remains production-authoritative;
+/// Flat is enabled only by differential tests until dual publication and the
+/// connected shadow gate complete.
+/// </summary>
+internal enum CollisionTraversalMode
+{
+    Graph,
+    Flat,
+}
+
+/// <summary>
+/// Narrow representation switch used while the graph and flat collision
+/// worlds coexist. I6 removes the graph branch after acceptance.
+/// </summary>
+internal static class CollisionTraversal
+{
+    internal static bool HasCellContainment(
+        PhysicsDataCache cache,
+        CellPhysics cell)
+    {
+        if (cache.CollisionTraversalMode == CollisionTraversalMode.Flat)
+        {
+            FlatCellContainmentBsp flat = cell.FlatContainmentBsp ??
+                throw MissingFlat("cell containment");
+            return flat.RootIndex >= 0;
+        }
+
+        return cell.CellBSP?.Root is not null;
+    }
+
+    internal static bool HasPhysics(
+        PhysicsDataCache cache,
+        CellPhysics cell)
+    {
+        if (cache.CollisionTraversalMode == CollisionTraversalMode.Flat)
+        {
+            FlatPhysicsBsp flat = cell.FlatPhysicsBsp ??
+                throw MissingFlat("cell physics");
+            return flat.RootIndex >= 0;
+        }
+
+        return cell.BSP?.Root is not null;
+    }
+
+    internal static bool HasPhysics(
+        PhysicsDataCache cache,
+        GfxObjPhysics gfxObject)
+    {
+        if (cache.CollisionTraversalMode == CollisionTraversalMode.Flat)
+        {
+            FlatPhysicsBsp flat = gfxObject.FlatPhysicsBsp ??
+                throw MissingFlat("GfxObj physics");
+            return flat.RootIndex >= 0;
+        }
+
+        return gfxObject.BSP.Root is not null;
+    }
+
+    internal static FlatCollisionSphere RootBoundingSphere(
+        PhysicsDataCache cache,
+        CellPhysics cell)
+    {
+        if (cache.CollisionTraversalMode == CollisionTraversalMode.Flat)
+        {
+            FlatPhysicsBsp flat = cell.FlatPhysicsBsp ??
+                throw MissingFlat("cell physics");
+            return flat.Nodes[flat.RootIndex].BoundingSphere;
+        }
+
+        DatReaderWriter.Types.Sphere sphere = cell.BSP!.Root!.BoundingSphere;
+        return new FlatCollisionSphere(sphere.Origin, sphere.Radius);
+    }
+
+    internal static bool PointInsideCell(
+        PhysicsDataCache cache,
+        CellPhysics cell,
+        Vector3 localPoint)
+    {
+        if (cache.CollisionTraversalMode == CollisionTraversalMode.Flat)
+        {
+            FlatCellContainmentBsp flat = cell.FlatContainmentBsp ??
+                throw MissingFlat("cell containment");
+            return FlatBspQuery.PointInsideCellBsp(flat, localPoint);
+        }
+
+        return BSPQuery.PointInsideCellBsp(cell.CellBSP?.Root, localPoint);
+    }
+
+    internal static bool SphereIntersectsCell(
+        PhysicsDataCache cache,
+        CellPhysics cell,
+        Vector3 localCenter,
+        float radius)
+    {
+        if (cache.CollisionTraversalMode == CollisionTraversalMode.Flat)
+        {
+            FlatCellContainmentBsp flat = cell.FlatContainmentBsp ??
+                throw MissingFlat("cell containment");
+            return FlatBspQuery.SphereIntersectsCellBsp(
+                flat,
+                localCenter,
+                radius);
+        }
+
+        return BSPQuery.SphereIntersectsCellBsp(
+            cell.CellBSP?.Root,
+            localCenter,
+            radius);
+    }
+
+    internal static TransitionState FindCollisions(
+        PhysicsDataCache cache,
+        CellPhysics cell,
+        Transition transition,
+        Vector3 localSphereCenter,
+        float localSphereRadius,
+        bool hasLocalSphere1,
+        Vector3 localSphere1Center,
+        float localSphere1Radius,
+        Vector3 localCurrentCenter,
+        Vector3 localSpaceZ,
+        float scale,
+        Quaternion localToWorld,
+        PhysicsEngine? engine,
+        Vector3 worldOrigin)
+    {
+        if (cache.CollisionTraversalMode == CollisionTraversalMode.Flat)
+        {
+            FlatPhysicsBsp flat = cell.FlatPhysicsBsp ??
+                throw MissingFlat("cell physics");
+            return FlatBspQuery.FindCollisions(
+                flat,
+                transition,
+                localSphereCenter,
+                localSphereRadius,
+                hasLocalSphere1,
+                localSphere1Center,
+                localSphere1Radius,
+                localCurrentCenter,
+                localSpaceZ,
+                scale,
+                localToWorld,
+                engine,
+                worldOrigin);
+        }
+
+        return BSPQuery.FindCollisions(
+            cell.BSP?.Root,
+            cell.Resolved,
+            transition,
+            localSphereCenter,
+            localSphereRadius,
+            hasLocalSphere1,
+            localSphere1Center,
+            localSphere1Radius,
+            localCurrentCenter,
+            localSpaceZ,
+            scale,
+            localToWorld,
+            engine,
+            worldOrigin);
+    }
+
+    internal static TransitionState FindCollisions(
+        PhysicsDataCache cache,
+        GfxObjPhysics gfxObject,
+        Transition transition,
+        Vector3 localSphereCenter,
+        float localSphereRadius,
+        bool hasLocalSphere1,
+        Vector3 localSphere1Center,
+        float localSphere1Radius,
+        Vector3 localCurrentCenter,
+        Vector3 localSpaceZ,
+        float scale,
+        Quaternion localToWorld,
+        PhysicsEngine? engine,
+        Vector3 worldOrigin)
+    {
+        if (cache.CollisionTraversalMode == CollisionTraversalMode.Flat)
+        {
+            FlatPhysicsBsp flat = gfxObject.FlatPhysicsBsp ??
+                throw MissingFlat("GfxObj physics");
+            return FlatBspQuery.FindCollisions(
+                flat,
+                transition,
+                localSphereCenter,
+                localSphereRadius,
+                hasLocalSphere1,
+                localSphere1Center,
+                localSphere1Radius,
+                localCurrentCenter,
+                localSpaceZ,
+                scale,
+                localToWorld,
+                engine,
+                worldOrigin);
+        }
+
+        return BSPQuery.FindCollisions(
+            gfxObject.BSP.Root,
+            gfxObject.Resolved,
+            transition,
+            localSphereCenter,
+            localSphereRadius,
+            hasLocalSphere1,
+            localSphere1Center,
+            localSphere1Radius,
+            localCurrentCenter,
+            localSpaceZ,
+            scale,
+            localToWorld,
+            engine,
+            worldOrigin);
+    }
+
+    private static InvalidOperationException MissingFlat(string kind) =>
+        new(
+            $"Flat collision traversal requires a prepared {kind} asset. " +
+            "Gameplay must not fall back silently.");
+}

src/AcDream.Core/Physics/PhysicsDataCache.cs

@@ -22,6 +22,14 @@ public sealed class PhysicsDataCache
     private readonly ConcurrentDictionary<uint, SetupPhysics> _setup = new();
     private readonly ConcurrentDictionary<uint, CellPhysics> _cellStruct = new();
 
+    /// <summary>
+    /// Slice I graph/flat differential selector. Production remains on the
+    /// graph oracle until I6; tests use the flat value to run the same complete
+    /// resolver from cloned inputs.
+    /// </summary>
+    internal CollisionTraversalMode CollisionTraversalMode { get; set; } =
+        CollisionTraversalMode.Graph;
+
     // ── Phase 2: building portal cache for outdoor→indoor entry ───────────
     private readonly ConcurrentDictionary<uint, BuildingPhysics> _buildings = new();
 
@@ -541,6 +549,12 @@ public sealed class GfxObjPhysics
     /// Populated once at cache time so BSP queries don't pay per-test lookup cost.
     /// </summary>
     public required Dictionary<ushort, ResolvedPolygon> Resolved { get; init; }
+
+    /// <summary>
+    /// Prepared integer-indexed shadow representation. Optional until Slice I5
+    /// dual publication is complete.
+    /// </summary>
+    public FlatPhysicsBsp? FlatPhysicsBsp { get; init; }
 }
 
 /// <summary>Cached collision shape data for a Setup (character/creature capsule).</summary>
@@ -578,6 +592,11 @@ public sealed class CellPhysics
     /// </summary>
     public required Dictionary<ushort, ResolvedPolygon> Resolved { get; init; }
 
+    /// <summary>
+    /// Prepared integer-indexed physics BSP shadow. Optional until Slice I5.
+    /// </summary>
+    public FlatPhysicsBsp? FlatPhysicsBsp { get; init; }
+
     // ── Indoor walking Phase 2 (2026-05-19): portal-graph fields ───────
 
     /// <summary>
@@ -590,6 +609,12 @@ public sealed class CellPhysics
     /// </summary>
     public DatReaderWriter.Types.CellBSPTree? CellBSP { get; init; }
 
+    /// <summary>
+    /// Prepared integer-indexed cell-containment BSP shadow. Optional until
+    /// Slice I5.
+    /// </summary>
+    public FlatCellContainmentBsp? FlatContainmentBsp { get; init; }
+
     /// <summary>
     /// Portal connections to neighbouring cells, in cell-local space.
     /// Default: empty list. Source: <c>envCell.CellPortals</c>.

src/AcDream.Core/Physics/TransitionTypes.cs

@@ -2298,7 +2298,11 @@ public sealed class Transition
 
                 var cell = engine.DataCache.GetCellStruct(cellId);
                 // R2 guard: stale CellPhysics loaded for render but not physics.
-                if (cell?.BSP?.Root is null) continue;
+                if (cell is null ||
+                    !CollisionTraversal.HasPhysics(engine.DataCache, cell))
+                {
+                    continue;
+                }
 
                 // Transform sphere into THIS cell's local space. Mirrors the
                 // primary-cell pattern at TransitionTypes.cs (FindEnvCollisions,
@@ -2330,8 +2334,8 @@ public sealed class Transition
                 if (PhysicsDiagnostics.ProbeIndoorBspEnabled)
                     PhysicsDiagnostics.LastBspHitPoly = null;
 
-                var result = BSPQuery.FindCollisions(
+                var result = CollisionTraversal.FindCollisions(
-                    cell.BSP.Root, cell.Resolved, this,
+                    engine.DataCache!, cell, this,
                     localCenter,
                     sphereRadius,
                     hasLocalSphere1,
@@ -2348,7 +2352,10 @@ public sealed class Transition
                         ? "poly=n/a"
                         : System.FormattableString.Invariant(
                             $"poly=0x{hit.Id:X4} n=({hit.Plane.Normal.X:F3},{hit.Plane.Normal.Y:F3},{hit.Plane.Normal.Z:F3}) d={hit.Plane.D:F3} sides={hit.SidesType}");
-                    var bs = cell.BSP.Root.BoundingSphere;
+                    FlatCollisionSphere bs =
+                        CollisionTraversal.RootBoundingSphere(
+                            engine.DataCache,
+                            cell);
                     string bsDesc = System.FormattableString.Invariant(
                         $"bs=({bs.Origin.X:F3},{bs.Origin.Y:F3},{bs.Origin.Z:F3}) br={bs.Radius:F3}");
                     Console.WriteLine(System.FormattableString.Invariant(
@@ -2620,7 +2627,8 @@ public sealed class Transition
         if (cellLow >= 0x0100 && engine.DataCache is not null)
         {
             var cellPhysics = engine.DataCache.GetCellStruct(sp.CheckCellId);
-            if (cellPhysics?.BSP?.Root is not null)
+            if (cellPhysics is not null &&
+                CollisionTraversal.HasPhysics(engine.DataCache!, cellPhysics))
             {
                 // Transform player sphere to cell-local space.
                 var localCenter = Vector3.Transform(footCenter, cellPhysics.InverseWorldTransform);
@@ -2677,9 +2685,9 @@ public sealed class Transition
                     cellOrigin = cellPhysics.WorldTransform.Translation;
                 }
 
-                var cellState = BSPQuery.FindCollisions(
+                var cellState = CollisionTraversal.FindCollisions(
-                    cellPhysics.BSP.Root,
+                    engine.DataCache!,
-                    cellPhysics.Resolved,
+                    cellPhysics,
                     this,
                     localCenter,
                     sphereRadius,
@@ -3134,12 +3142,14 @@ public sealed class Transition
                 {
                     Vector3 dxy = obj.Position - sp.GlobalCurrCenter[0].Origin;
                     float distXY = MathF.Sqrt(dxy.X * dxy.X + dxy.Y * dxy.Y);
-                    bool cacheHit = physics?.BSP?.Root is not null;
+                    bool cacheHit = physics is not null &&
+                        CollisionTraversal.HasPhysics(engine.DataCache!, physics);
                     Console.WriteLine(System.FormattableString.Invariant(
                         $"[bsp-test] obj=0x{obj.EntityId:X8} gfx=0x{obj.GfxObjId:X8} state=0x{obj.State:X8} radius={obj.Radius:F3} pos=({obj.Position.X:F2},{obj.Position.Y:F2},{obj.Position.Z:F2}) distXY={distXY:F3} cacheHit={cacheHit}"));
                 }
 
-                if (physics?.BSP?.Root is null)
+                if (physics is null ||
+                    !CollisionTraversal.HasPhysics(engine.DataCache!, physics))
                 {
                     // Clear obstruction_ethereal before skipping — retail's per-object
                     // clear (pc:276989) fires after shape tests; we clear early here to
@@ -3179,9 +3189,9 @@ public sealed class Transition
                 // Use the retail 6-path dispatcher with pre-resolved polygons.
                 // Pass the object's scale so collision response offsets (in
                 // unscaled local space) are multiplied back to world space.
-                result = BSPQuery.FindCollisions(
+                result = CollisionTraversal.FindCollisions(
-                    physics.BSP.Root,
+                    engine.DataCache!,
-                    physics.Resolved,
+                    physics,
                     this,
                     localSphere0Center,
                     localSphere0Radius,
@@ -3476,7 +3486,11 @@ public sealed class Transition
         // CacheBuilding site resolves 0x02 Setup models to their first part
         // — retail tests part_array->parts[0] only, 0x006b5320).
         var physics = engine.DataCache.GetGfxObj(building.ModelId);
-        if (physics?.BSP?.Root is null) return TransitionState.OK;
+        if (physics is null ||
+            !CollisionTraversal.HasPhysics(engine.DataCache, physics))
+        {
+            return TransitionState.OK;
+        }
 
         var sp = SpherePath;
         var ci = CollisionInfo;
@@ -3509,9 +3523,9 @@ public sealed class Transition
         TransitionState result;
         try
         {
-            result = BSPQuery.FindCollisions(
+            result = CollisionTraversal.FindCollisions(
-                physics.BSP.Root,
+                engine.DataCache!,
-                physics.Resolved,
+                physics,
                 this,
                 localSphere0Center,
                 localSphere0Radius,