feat(physics): complete retail collision — indoor BSP, dual sphere, step-up, swept-sphere, 6-path dispatcher

Indoor CellStruct PhysicsBSP collision for room walls/ceilings.
Dual sphere (body+head) from Setup dimensions.
StepUp attempts before sliding when hitting low obstacles.
FindTimeOfCollision for exact parametric BSP contact time.
Full 6-path BSP dispatcher wired into FindEnvCollisions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/AcDream.Core/Physics/PhysicsDataCache.cs

@@ -1,4 +1,5 @@
 using System.Collections.Concurrent;
+using System.Numerics;
 using DatReaderWriter.DBObjs;
 using DatReaderWriter.Enums;
 using DatReaderWriter.Types;
@@ -15,6 +16,7 @@ public sealed class PhysicsDataCache
 {
     private readonly ConcurrentDictionary<uint, GfxObjPhysics> _gfxObj = new();
     private readonly ConcurrentDictionary<uint, SetupPhysics> _setup = new();
+    private readonly ConcurrentDictionary<uint, CellPhysics> _cellStruct = new();
 
     /// <summary>
     /// Extract and cache the physics BSP + polygon data from a GfxObj.
@@ -53,10 +55,35 @@ public sealed class PhysicsDataCache
         };
     }
 
+    /// <summary>
+    /// Extract and cache the physics BSP + polygon data from a CellStruct
+    /// (indoor room geometry). No-ops if the id is already cached or the
+    /// CellStruct has no physics BSP.
+    /// </summary>
+    public void CacheCellStruct(uint envCellId, CellStruct cellStruct,
+                                 Matrix4x4 worldTransform)
+    {
+        if (_cellStruct.ContainsKey(envCellId)) return;
+        if (cellStruct.PhysicsBSP?.Root is null) return;
+
+        Matrix4x4.Invert(worldTransform, out var inverseTransform);
+
+        _cellStruct[envCellId] = new CellPhysics
+        {
+            BSP = cellStruct.PhysicsBSP,
+            PhysicsPolygons = cellStruct.PhysicsPolygons,
+            Vertices = cellStruct.VertexArray,
+            WorldTransform = worldTransform,
+            InverseWorldTransform = inverseTransform,
+        };
+    }
+
     public GfxObjPhysics? GetGfxObj(uint id) => _gfxObj.TryGetValue(id, out var p) ? p : null;
     public SetupPhysics? GetSetup(uint id) => _setup.TryGetValue(id, out var p) ? p : null;
+    public CellPhysics? GetCellStruct(uint id) => _cellStruct.TryGetValue(id, out var p) ? p : null;
     public int GfxObjCount => _gfxObj.Count;
     public int SetupCount => _setup.Count;
+    public int CellStructCount => _cellStruct.Count;
 }
 
 /// <summary>Cached physics data for a single GfxObj part.</summary>
@@ -78,3 +105,17 @@ public sealed class SetupPhysics
     public float StepUpHeight { get; init; }
     public float StepDownHeight { get; init; }
 }
+
+/// <summary>
+/// Cached physics data for an indoor cell's room geometry (CellStruct).
+/// Used for wall/floor/ceiling collision in EnvCells.
+/// ACE: EnvCell.find_env_collisions queries CellStructure.PhysicsBSP.
+/// </summary>
+public sealed class CellPhysics
+{
+    public required PhysicsBSPTree BSP { get; init; }
+    public required Dictionary<ushort, Polygon> PhysicsPolygons { get; init; }
+    public required VertexArray Vertices { get; init; }
+    public Matrix4x4 WorldTransform { get; init; }
+    public Matrix4x4 InverseWorldTransform { get; init; }
+}