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/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