feat(phys L.2a slice 1): resolver + cell-transit probes (PhysicsDiagnostics)

New static `AcDream.Core.Physics.PhysicsDiagnostics` holds two
runtime-toggleable flags initialized from env vars:

- ACDREAM_PROBE_RESOLVE=1 — emit one [resolve] line per
  PhysicsEngine.ResolveWithTransition call: input/target/output
  position+cell, ok-vs-partial, grounded-in, contact-plane status,
  wall normal if hit, walkable-polygon valid, moving entity id.
- ACDREAM_PROBE_CELL=1 — emit one [cell-transit] line per
  PlayerMovementController.CellId change: old → new cell, current
  world position, reason tag (resolver / teleport).

Both also exposed as runtime-toggleable checkboxes in the DebugPanel
"Diagnostics" section. Unlike the existing four Dump-* checkboxes
(which only mirror sticky-at-startup env vars), the two new ones
forward directly to PhysicsDiagnostics — toggling on/off takes
effect on the next physics resolve, no relaunch.

Why now: L.2's plan-of-record (docs/plans/2026-04-29-movement-collision-
conformance.md) explicitly says "Land L.2a diagnostics first. Do not
make another physics change blind." This slice closes the most-load-
bearing gap in L.2a — a general-purpose probe on the resolver outcome
and a cell-transit log — so that later L.2b/c/d/e physics changes can
be evidence-driven instead of guessed. Foundation for the indoor /
dungeon walking trajectory (G.3 unblock).

Pure additive: when both flags are off (default), the probes collapse
to a single static-bool read per resolve, zero log cost. PlayerMovement
Controller's two CellId-mutation sites are now routed through a
private UpdateCellId(reason) helper for diag chokepoint.

Build green, 1032/1040 unit tests pass. The 8 failing tests are
pre-existing on the branch base (verified by stash-and-rerun);
none touch resolver or cell-transit code; all fail identically with
this slice stashed. Investigation deferred to a follow-up.

Refs: docs/plans/2026-04-29-movement-collision-conformance.md (L.2a
shipped-slice note added in same commit).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/plans/2026-04-29-movement-collision-conformance.md

@@ -92,6 +92,35 @@ Goal: make every bad movement outcome explainable.
 - Build real-DAT fixture capture for known walls, building ledges, rooftops,
   slopes, landblock seams, and dungeon entrances.
 
+Current shipped slices:
+
+- 2026-04-30: cdb + TTD retail-observer toolchain (`tools/pdb-extract/`,
+  `tools/ttd-record.ps1`, `tools/ttd-query.ps1`) with PDB pairing checker
+  and ring-buffer trace replay. The "retail observer harness" line item.
+- 2026-04 (pre-L.2 rename): `ACDREAM_DUMP_MOVE_TRUTH` paired
+  outbound/server-echo dumper in `GameWindow` covers outbound packet
+  fields + server echo + correction delta with cell-id mismatch.
+- Pre-L.2: scenario-specific dumps `ACDREAM_DUMP_MOTION`,
+  `ACDREAM_DUMP_STEEP_ROOF`, `ACDREAM_DUMP_STEPUP`,
+  `ACDREAM_DUMP_EDGE_SLIDE` for the codepaths hit during prior bug chases.
+- 2026-05-12 (slice 1): general-purpose probes via new
+  `AcDream.Core.Physics.PhysicsDiagnostics` static class.
+  `ACDREAM_PROBE_RESOLVE` emits one `[resolve]` line per
+  `PhysicsEngine.ResolveWithTransition` call (input/output pos+cell,
+  ok-vs-partial, grounded-in, contact-plane status, wall normal if hit,
+  walkable polygon valid, moving entity id).
+  `ACDREAM_PROBE_CELL` emits one `[cell-transit]` line per
+  `PlayerMovementController.CellId` change with old→new + position +
+  reason tag (`resolver`/`teleport`). Both flippable live via the
+  DebugPanel "Diagnostics" section — checkbox toggles take effect on
+  the next resolve, no relaunch required.
+
+Remaining L.2a work: contact-plane probe (general, not just steep-roof),
+ShadowObjectRegistry hit log ("you collided with entity X"), water probe,
+real-DAT fixture-capture pipeline, and folding the older sticky-at-startup
+`ACDREAM_DUMP_*` flags into `PhysicsDiagnostics` for unified runtime
+toggling.
+
 ### L.2b - Movement Wire / Contact Authority
 
 Goal: stop sending movement packets that claim more certainty than the local

src/AcDream.App/Input/PlayerMovementController.cs

@@ -288,12 +288,29 @@ public sealed class PlayerMovementController
         _motion.apply_current_movement(cancelMoveTo: false, allowJump: false);
     }
 
+    // L.2a slice 1 (2026-05-12): centralized CellId mutation so the
+    // [cell-transit] probe fires from a single chokepoint. Both the
+    // server-snap path (SetPosition) and the per-frame resolver path
+    // route through here. When PhysicsDiagnostics.ProbeCellEnabled is
+    // off this collapses to a single bool-compare + assignment — zero
+    // logging cost.
+    private void UpdateCellId(uint newCellId, string reason)
+    {
+        if (newCellId != CellId && PhysicsDiagnostics.ProbeCellEnabled)
+        {
+            var pos = _body.Position;
+            Console.WriteLine(System.FormattableString.Invariant(
+                $"[cell-transit] 0x{CellId:X8} -> 0x{newCellId:X8} pos=({pos.X:F3},{pos.Y:F3},{pos.Z:F3}) reason={reason}"));
+        }
+        CellId = newCellId;
+    }
+
     public void SetPosition(Vector3 pos, uint cellId)
     {
         _body.Position = pos;
         _prevPhysicsPos = pos;
         _currPhysicsPos = pos;
-        CellId = cellId;
+        UpdateCellId(cellId, "teleport");
 
         // Treat as grounded after a server-side position snap.
         _body.TransientState = TransientStateFlags.Contact | TransientStateFlags.OnWalkable;
@@ -760,7 +777,7 @@ public sealed class PlayerMovementController
 
 
         _wasAirborneLastFrame = !_body.OnWalkable;
-        CellId = resolveResult.CellId;
+        UpdateCellId(resolveResult.CellId, "resolver");
 
         // ── 6. Determine outbound motion commands ─────────────────────────────
         uint? outForwardCmd = null;

src/AcDream.Core/Physics/PhysicsDiagnostics.cs

@@ -0,0 +1,40 @@
+using System;
+
+namespace AcDream.Core.Physics;
+
+/// <summary>
+/// L.2a slice 1 (2026-05-12) — runtime-toggleable physics probe flags.
+/// Initialized from env vars at process start; flippable at runtime via
+/// the DebugPanel mirror (or by direct assignment). Log call sites read
+/// these statics so a checkbox toggle takes effect on the next resolve
+/// without relaunching.
+///
+/// <para>
+/// Slice 1 ships <see cref="ProbeResolveEnabled"/> +
+/// <see cref="ProbeCellEnabled"/>. Future slices may fold the older
+/// <c>ACDREAM_DUMP_*</c> env vars into this class for unified runtime
+/// toggling. Until then, those older flags remain sticky-at-startup
+/// per their original implementation.
+/// </para>
+/// </summary>
+public static class PhysicsDiagnostics
+{
+    /// <summary>
+    /// When true, <see cref="PhysicsEngine.ResolveWithTransition"/> emits
+    /// one structured <c>[resolve]</c> line per call: input + target +
+    /// output position/cell, grounded state, contact-plane status,
+    /// collision-normal validity, walkable polygon status, moving entity
+    /// id. Initial state from <c>ACDREAM_PROBE_RESOLVE=1</c>.
+    /// </summary>
+    public static bool ProbeResolveEnabled { get; set; } =
+        Environment.GetEnvironmentVariable("ACDREAM_PROBE_RESOLVE") == "1";
+
+    /// <summary>
+    /// When true, every change to <c>PlayerMovementController.CellId</c>
+    /// emits one <c>[cell-transit]</c> line: old → new cell, current
+    /// world position, reason tag (<c>resolver</c> / <c>teleport</c>).
+    /// Initial state from <c>ACDREAM_PROBE_CELL=1</c>.
+    /// </summary>
+    public static bool ProbeCellEnabled { get; set; } =
+        Environment.GetEnvironmentVariable("ACDREAM_PROBE_CELL") == "1";
+}

src/AcDream.Core/Physics/PhysicsEngine.cs

@@ -677,6 +677,28 @@ public sealed class PhysicsEngine
                 $"deltaXY=({dx:F3},{dy:F3}) cp={cpInfo}");
         }
 
+        // L.2a slice 1 (2026-05-12): general-purpose resolver probe.
+        // One line per call when PhysicsDiagnostics.ProbeResolveEnabled
+        // is set (env var ACDREAM_PROBE_RESOLVE=1 at startup, or the
+        // DebugPanel checkbox flipped at runtime). Captures every
+        // dimension L.2 cares about: input/output position, input/output
+        // cell, ok-vs-partial, grounded-in vs contact-out, contact-plane
+        // status, wall normal if hit, walkable polygon valid. Zero cost
+        // when off (one static-bool read).
+        if (PhysicsDiagnostics.ProbeResolveEnabled)
+        {
+            var probePost = sp.CheckPos;
+            string probeCp = ci.ContactPlaneValid
+                ? "valid"
+                : (ci.LastKnownContactPlaneValid ? "lastKnown" : "none");
+            string probeHit = collisionNormalValid
+                ? System.FormattableString.Invariant(
+                    $"yes n=({collisionNormal.X:F2},{collisionNormal.Y:F2},{collisionNormal.Z:F2})")
+                : "no";
+            Console.WriteLine(System.FormattableString.Invariant(
+                $"[resolve] ent=0x{movingEntityId:X8} in=({currentPos.X:F3},{currentPos.Y:F3},{currentPos.Z:F3}) cell=0x{cellId:X8} tgt=({targetPos.X:F3},{targetPos.Y:F3},{targetPos.Z:F3}) out=({probePost.X:F3},{probePost.Y:F3},{probePost.Z:F3}) cell=0x{sp.CheckCellId:X8} ok={ok} groundedIn={isOnGround} cp={probeCp} hit={probeHit} walkable={sp.HasLastWalkablePolygon}"));
+        }
+
         if (ok)
         {
             bool onGround = ci.ContactPlaneValid

src/AcDream.UI.Abstractions/Panels/Debug/DebugPanel.cs

@@ -203,11 +203,17 @@ public sealed class DebugPanel : IPanel
         bool dumpVitals   = _vm.DumpVitals;
         bool dumpOpcodes  = _vm.DumpOpcodes;
         bool dumpSky      = _vm.DumpSky;
+        bool probeResolve = _vm.ProbeResolve;
+        bool probeCell    = _vm.ProbeCell;
 
         if (r.Checkbox("Dump motion (ACDREAM_DUMP_MOTION)",     ref dumpMotion))   _vm.DumpMotion   = dumpMotion;
         if (r.Checkbox("Dump vitals (ACDREAM_DUMP_VITALS)",     ref dumpVitals))   _vm.DumpVitals   = dumpVitals;
         if (r.Checkbox("Dump opcodes (ACDREAM_DUMP_OPCODES)",   ref dumpOpcodes))  _vm.DumpOpcodes  = dumpOpcodes;
         if (r.Checkbox("Dump sky (ACDREAM_DUMP_SKY)",           ref dumpSky))      _vm.DumpSky      = dumpSky;
+        // L.2a slice 1 (2026-05-12): unlike the four above, these
+        // forward to PhysicsDiagnostics so a toggle takes effect live.
+        if (r.Checkbox("Probe resolve (ACDREAM_PROBE_RESOLVE)",  ref probeResolve)) _vm.ProbeResolve = probeResolve;
+        if (r.Checkbox("Probe cell-transit (ACDREAM_PROBE_CELL)",ref probeCell))    _vm.ProbeCell    = probeCell;
 
         r.Spacing();
 

src/AcDream.UI.Abstractions/Panels/Debug/DebugVM.cs

@@ -1,5 +1,6 @@
 using System.Numerics;
 using AcDream.Core.Combat;
+using AcDream.Core.Physics;
 
 namespace AcDream.UI.Abstractions.Panels.Debug;
 
@@ -234,6 +235,32 @@ public sealed class DebugVM
     /// <summary>Mirror of <c>ACDREAM_DUMP_SKY</c>.</summary>
     public bool DumpSky { get; set; }
 
+    // L.2a slice 1 (2026-05-12): unlike DumpMotion/Vitals/Opcodes/Sky
+    // above (which are display-only mirrors of sticky-at-startup env
+    // vars), these forward directly to the PhysicsDiagnostics statics,
+    // so checkbox toggles take effect on the next physics resolve.
+    /// <summary>
+    /// Runtime mirror of <c>PhysicsDiagnostics.ProbeResolveEnabled</c>
+    /// (env var <c>ACDREAM_PROBE_RESOLVE</c>). Toggling here flips the
+    /// resolver probe live — no relaunch required.
+    /// </summary>
+    public bool ProbeResolve
+    {
+        get => PhysicsDiagnostics.ProbeResolveEnabled;
+        set => PhysicsDiagnostics.ProbeResolveEnabled = value;
+    }
+
+    /// <summary>
+    /// Runtime mirror of <c>PhysicsDiagnostics.ProbeCellEnabled</c>
+    /// (env var <c>ACDREAM_PROBE_CELL</c>). Toggling here flips the
+    /// cell-transit probe live.
+    /// </summary>
+    public bool ProbeCell
+    {
+        get => PhysicsDiagnostics.ProbeCellEnabled;
+        set => PhysicsDiagnostics.ProbeCellEnabled = value;
+    }
+
     // ── Action hooks invoked by panel buttons ──────────────────────────
 
     /// <summary>