feat(render #53): EntityClassificationCache.InvalidateLandblock + tests

Sweep-by-landblock removal for the streaming demote/unload path. Tests
#6, #7, #8 from spec section 7.1 lock in: (a) all matching entries removed,
(b) non-matching entries preserved, (c) idempotent on missing LB.

Phase 1 (cache foundation) complete. 11 cache tests passing.

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

src/AcDream.App/Rendering/Wb/EntityClassificationCache.cs

@@ -71,4 +71,30 @@ internal sealed class EntityClassificationCache
     {
         _entries.Remove(entityId);
     }
+
+    /// <summary>
+    /// Remove every cache entry whose <see cref="EntityCacheEntry.LandblockHint"/>
+    /// equals <paramref name="landblockId"/>. Used by the streaming pipeline
+    /// when a landblock demotes from near to far or unloads. No-op if no
+    /// entries match.
+    /// </summary>
+    public void InvalidateLandblock(uint landblockId)
+    {
+        if (_entries.Count == 0) return;
+
+        // Collect the ids to remove first to avoid mutating the dict during iteration.
+        // Buffered locally because the typical case removes ~all entries in the LB
+        // (which is still small relative to the total cache).
+        List<uint>? toRemove = null;
+        foreach (var (id, entry) in _entries)
+        {
+            if (entry.LandblockHint == landblockId)
+            {
+                toRemove ??= new List<uint>();
+                toRemove.Add(id);
+            }
+        }
+        if (toRemove is null) return;
+        foreach (var id in toRemove) _entries.Remove(id);
+    }
 }