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>

tests/AcDream.Core.Tests/Physics/RemoveCellsForLandblockTests.cs

@@ -0,0 +1,66 @@
+using System.Collections.Generic;
+using AcDream.Core.Physics;
+using Xunit;
+
+namespace AcDream.Core.Tests.Physics;
+
+/// <summary>
+/// D8 (2026-06-24): verify that <see cref="PhysicsDataCache.RemoveCellsForLandblock"/>
+/// evicts every cached cell belonging to the given landblock prefix and leaves
+/// cells belonging to other landblocks untouched — symmetric with
+/// <see cref="PhysicsDataCache.RemoveBuildingsForLandblock"/> (#146).
+/// </summary>
+public class RemoveCellsForLandblockTests
+{
+    /// <summary>
+    /// Build the absolute minimum <see cref="CellPhysics"/> that satisfies the
+    /// <c>required</c> init property. BSP is left null because the test exercises
+    /// only the cache-eviction key logic, not BSP traversal.
+    /// </summary>
+    private static CellPhysics BuildMinimalCell() => new()
+    {
+        Resolved = new Dictionary<ushort, ResolvedPolygon>(),
+    };
+
+    [Fact]
+    public void RemoveCellsForLandblock_EvictsOnlyMatchingPrefix()
+    {
+        var cache = new PhysicsDataCache();
+
+        // Two cells from landblock 0xAAAA0000 and one from 0xBBBB0000.
+        cache.RegisterCellStructForTest(0xAAAA0100u, BuildMinimalCell());
+        cache.RegisterCellStructForTest(0xAAAA0200u, BuildMinimalCell());
+        cache.RegisterCellStructForTest(0xBBBB0100u, BuildMinimalCell());
+
+        cache.RemoveCellsForLandblock(0xAAAA0000u);
+
+        // Both AAAA cells must be gone.
+        Assert.Null(cache.GetCellStruct(0xAAAA0100u));
+        Assert.Null(cache.GetCellStruct(0xAAAA0200u));
+
+        // The BBBB cell must survive.
+        Assert.NotNull(cache.GetCellStruct(0xBBBB0100u));
+    }
+
+    [Fact]
+    public void RemoveCellsForLandblock_EmptyCache_DoesNotThrow()
+    {
+        // Must be a no-op on an empty cache.
+        var cache = new PhysicsDataCache();
+        cache.RemoveCellsForLandblock(0xAAAA0000u);
+        Assert.Equal(0, cache.CellStructCount);
+    }
+
+    [Fact]
+    public void RemoveCellsForLandblock_NoMatchingCells_LeavesOthersIntact()
+    {
+        var cache = new PhysicsDataCache();
+        cache.RegisterCellStructForTest(0xBBBB0100u, BuildMinimalCell());
+
+        // Evicting a prefix that has no cached entries should not disturb others.
+        cache.RemoveCellsForLandblock(0xAAAA0000u);
+
+        Assert.NotNull(cache.GetCellStruct(0xBBBB0100u));
+        Assert.Equal(1, cache.CellStructCount);
+    }
+}