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>

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>.