feat(A.5 T8): WorldEntity AABB cache + dirty flag

Adds AabbMin/AabbMax (per-entity world-space bounding box) and AabbDirty
flag to WorldEntity. RefreshAabb() recomputes the box from Position ±5 m
(DefaultAabbRadius). SetPosition() writes Position and marks the cache
dirty so the dispatcher calls RefreshAabb on first read rather than
carrying stale bounds.

AabbDirty defaults to true on construction — freshly-built entities have
zero AabbMin/AabbMax until RefreshAabb is called. Two new conformance tests
verify the ±5 m geometry and the dirty/clean state machine.

Per Phase A.5 spec §4.6 Change #2.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/AcDream.Core.Tests/World/WorldEntityAabbTests.cs

@@ -0,0 +1,47 @@
+using System.Numerics;
+using AcDream.Core.World;
+using Xunit;
+
+namespace AcDream.Core.Tests.World;
+
+public class WorldEntityAabbTests
+{
+    [Fact]
+    public void Aabb_DefaultRadius_PositionPlusMinus5()
+    {
+        var entity = new WorldEntity
+        {
+            Id = 1,
+            SourceGfxObjOrSetupId = 0,
+            Position = new Vector3(10, 20, 30),
+            Rotation = System.Numerics.Quaternion.Identity,
+            MeshRefs = System.Array.Empty<MeshRef>(),
+        };
+        entity.RefreshAabb();
+
+        Assert.Equal(new Vector3(5, 15, 25), entity.AabbMin);
+        Assert.Equal(new Vector3(15, 25, 35), entity.AabbMax);
+    }
+
+    [Fact]
+    public void Aabb_DirtyFlag_SetByMutator_ClearedByRefresh()
+    {
+        var entity = new WorldEntity
+        {
+            Id = 1,
+            SourceGfxObjOrSetupId = 0,
+            Position = new Vector3(10, 20, 30),
+            Rotation = System.Numerics.Quaternion.Identity,
+            MeshRefs = System.Array.Empty<MeshRef>(),
+        };
+        entity.RefreshAabb();
+        Assert.False(entity.AabbDirty);
+
+        entity.SetPosition(new Vector3(100, 200, 300));
+        Assert.True(entity.AabbDirty);
+
+        entity.RefreshAabb();
+        Assert.False(entity.AabbDirty);
+        Assert.Equal(new Vector3(95, 195, 295), entity.AabbMin);
+    }
+}