feat(physics): cell-based ShadowObject collision

Register static entities into terrain cells during streaming.
Transition system queries nearby objects and runs BSP collision.
Player can no longer walk through trees and buildings.

- ShadowObjectRegistry: 24m×24m cell index, Register/Deregister/
  RemoveLandblock/GetNearbyObjects matching retail AC's approach
- PhysicsEngine: ShadowObjects property + DataCache wiring point;
  RemoveLandblock now also clears shadow objects; TryGetLandblockContext
  helper lets Transition resolve landblock id+offset for a world pos
- Transition.FindObjCollisions: queries registry, broad-phase sphere test,
  narrow-phase BSPQuery.SphereIntersectsPoly in object-local space,
  returns Slid on hit to redirect movement along the surface
- GameWindow.ApplyLoadedTerrainLocked: registers each static entity after
  physics BSP data is cached; selects radius from BSP bounding sphere or
  Setup.Radius; wires PhysicsDataCache into engine on OnLoad
- 16 new ShadowObjectRegistry unit tests, all 361 tests green

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/AcDream.Core/Physics/PhysicsEngine.cs

@@ -24,6 +24,19 @@ public sealed class PhysicsEngine
     /// <summary>Number of registered landblocks (diagnostic).</summary>
     public int LandblockCount => _landblocks.Count;
 
+    /// <summary>
+    /// Cell-based spatial index for static object collision.
+    /// Populated during landblock streaming; queried by the Transition system.
+    /// </summary>
+    public ShadowObjectRegistry ShadowObjects { get; } = new();
+
+    /// <summary>
+    /// Physics BSP cache shared with the streaming loader. Set once by the
+    /// host (GameWindow) immediately after construction. The Transition system
+    /// reads this during FindObjCollisions to perform narrow-phase BSP tests.
+    /// </summary>
+    public PhysicsDataCache? DataCache { get; set; }
+
     private sealed record LandblockPhysics(
         TerrainSurface Terrain,
         IReadOnlyList<CellSurface> Cells,
@@ -43,9 +56,41 @@ public sealed class PhysicsEngine
     }
 
     /// <summary>
-    /// Remove a previously registered landblock.
+    /// Remove a previously registered landblock, including its shadow objects.
     /// </summary>
-    public void RemoveLandblock(uint landblockId) => _landblocks.Remove(landblockId);
+    public void RemoveLandblock(uint landblockId)
+    {
+        _landblocks.Remove(landblockId);
+        ShadowObjects.RemoveLandblock(landblockId);
+    }
+
+    /// <summary>
+    /// Find the landblock that contains the given world-space XY position and
+    /// return its ID plus world-space origin offsets. Returns false when no
+    /// registered landblock covers the position.
+    /// Used by Transition.FindObjCollisions to build the shadow-object query.
+    /// </summary>
+    public bool TryGetLandblockContext(float worldX, float worldY,
+        out uint landblockId, out float worldOffsetX, out float worldOffsetY)
+    {
+        foreach (var kvp in _landblocks)
+        {
+            var lb = kvp.Value;
+            float localX = worldX - lb.WorldOffsetX;
+            float localY = worldY - lb.WorldOffsetY;
+            if (localX >= 0f && localX < 192f && localY >= 0f && localY < 192f)
+            {
+                landblockId  = kvp.Key;
+                worldOffsetX = lb.WorldOffsetX;
+                worldOffsetY = lb.WorldOffsetY;
+                return true;
+            }
+        }
+        landblockId  = 0;
+        worldOffsetX = 0f;
+        worldOffsetY = 0f;
+        return false;
+    }
 
     /// <summary>
     /// Sample the outdoor terrain Z at the given world-space XY position.