feat(A.5 T14): GpuWorldState RemoveEntitiesFromLandblock + AddEntitiesToExisting

Two new methods on GpuWorldState, used by two-tier streaming (T13):

- RemoveEntitiesFromLandblock(id): drop all entities from an LB while
  keeping the terrain. Used for Near->Far demote (player walks past the
  inner ring; LB stays loaded but entities leave).
- AddEntitiesToExistingLandblock(id, entities): merge new entities into
  an already-loaded LB record. Used for Far->Near promote (terrain is
  already on the GPU; just streaming the entity layer in).

Falls back to the pending bucket if the LB hasn't loaded yet.

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

src/AcDream.App/Streaming/GpuWorldState.cs

@@ -339,6 +339,51 @@ public sealed class GpuWorldState
         bucket.Add(entity);
     }
 
+    /// <summary>
+    /// Drop all entities from a landblock without removing the terrain. Used
+    /// by two-tier streaming when a landblock crosses Near→Far hysteresis.
+    /// Per Phase A.5 spec §4.4.
+    /// </summary>
+    public void RemoveEntitiesFromLandblock(uint landblockId)
+    {
+        if (!_loaded.TryGetValue(landblockId, out var lb)) return;
+        if (_wbSpawnAdapter is not null)
+            _wbSpawnAdapter.OnLandblockUnloaded(landblockId);
+        _loaded[landblockId] = new LoadedLandblock(lb.LandblockId, lb.Heightmap, System.Array.Empty<WorldEntity>());
+        _pendingByLandblock.Remove(landblockId);
+        RebuildFlatView();
+    }
+
+    /// <summary>
+    /// Merge entities into an existing-loaded landblock. Used by two-tier
+    /// streaming for the Far→Near promotion case (terrain already loaded;
+    /// entity layer streaming in). Falls back to the pending bucket if the
+    /// landblock isn't loaded yet (handles the rare "promote arrives before
+    /// far load completes" race).
+    /// Per Phase A.5 spec §4.4.
+    /// </summary>
+    public void AddEntitiesToExistingLandblock(uint landblockId, System.Collections.Generic.IReadOnlyList<WorldEntity> entities)
+    {
+        if (!_loaded.TryGetValue(landblockId, out var lb))
+        {
+            // Park as pending — same pattern as AppendLiveEntity for not-yet-loaded LBs.
+            if (!_pendingByLandblock.TryGetValue(landblockId, out var bucket))
+            {
+                bucket = new List<WorldEntity>();
+                _pendingByLandblock[landblockId] = bucket;
+            }
+            bucket.AddRange(entities);
+            return;
+        }
+        var merged = new List<WorldEntity>(lb.Entities.Count + entities.Count);
+        merged.AddRange(lb.Entities);
+        merged.AddRange(entities);
+        _loaded[landblockId] = new LoadedLandblock(lb.LandblockId, lb.Heightmap, merged);
+        if (_wbSpawnAdapter is not null)
+            _wbSpawnAdapter.OnLandblockLoaded(_loaded[landblockId]);
+        RebuildFlatView();
+    }
+
     private void RebuildFlatView()
     {
         _flatEntities = _loaded.Values.SelectMany(lb => lb.Entities).ToArray();