fix(physics): D8 — RemoveCellsForLandblock; cell transforms rebase per apply

Mirrors RemoveBuildingsForLandblock (#146) for indoor CellPhysics entries.
_cellStruct is first-wins (CacheCellStruct's ContainsKey guard); without
eviction a dungeon's BSP WorldTransform is permanently locked to the
_liveCenter value at first streaming — a teleport recenter leaves cells at a
stale offset (~source↔dest distance), and foot-sphere collision queries miss
the geometry that visually renders correctly.

PhysicsDataCache.RemoveCellsForLandblock iterates _cellStruct.Keys and
TryRemove-s every entry whose high-word matches the evicted landblock prefix.
PhysicsEngine.RemoveLandblock now calls it alongside ShadowObjects.RemoveLandblock
so cell BSPs rebase on the next CacheCellStruct pass, same as buildings.

No divergence register row needed: this closes a gap introduced when the #146
building-eviction pattern was created without the symmetric cell eviction.

Tests: RemoveCellsForLandblockTests (3 cases): evicts-matching-prefix,
empty-cache no-throw, no-matching-cells leaves-others. Core suite: 1587 pass / 0 fail.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/AcDream.Core/Physics/PhysicsDataCache.cs

@@ -459,6 +459,24 @@ public sealed class PhysicsDataCache
                 _buildings.TryRemove(key, out _);
     }
 
+    /// <summary>
+    /// D8 (2026-06-24): drop every cached cell struct belonging to a landblock so
+    /// the next <see cref="CacheCellStruct"/> re-bases its STREAMING-RELATIVE
+    /// <c>WorldTransform</c> against the current <c>_liveCenter</c> — symmetric with
+    /// <see cref="RemoveBuildingsForLandblock"/> (#146). Without this, the
+    /// <c>_cellStruct</c> first-wins guard LOCKS a dungeon cell's transform at its
+    /// first streaming frame; a teleport recenter then leaves the cell BSP at a stale
+    /// world offset (~the source↔dest landblock distance), so foot-sphere collision
+    /// queries walk through where the wall geometry is.
+    /// </summary>
+    public void RemoveCellsForLandblock(uint landblockId)
+    {
+        uint prefix = landblockId & 0xFFFF0000u;
+        foreach (var key in _cellStruct.Keys)
+            if ((key & 0xFFFF0000u) == prefix)
+                _cellStruct.TryRemove(key, out _);
+    }
+
     public BuildingPhysics? GetBuilding(uint landcellId)
         => _buildings.TryGetValue(landcellId, out var b) ? b : null;
 

src/AcDream.Core/Physics/PhysicsEngine.cs

@@ -122,6 +122,7 @@ public sealed class PhysicsEngine
     {
         _landblocks.Remove(landblockId);
         ShadowObjects.RemoveLandblock(landblockId);
+        DataCache?.RemoveCellsForLandblock(landblockId);   // D8: rebase cell BSP transforms on next apply
 
         // #145: if the player's current cell belonged to the landblock being removed (a teleport
         // drops the stale source center via OnLivePositionUpdated), clear it. Otherwise CurrCell