feat(vfx #C.1.5b): GpuWorldState fires activator for dat-hydrated entities

Four new foreach blocks in GpuWorldState wire EntityScriptActivator
into the dat-hydration spawn/despawn paths:
- AddLandblock: fires OnCreate for each entity with ServerGuid==0
  (live entities filtered out — they got OnCreate at AppendLiveEntity
  and would double-fire on pending-bucket merges).
- AddEntitiesToExistingLandblock: fires OnCreate for each entity in
  the promoted batch (all dat-hydrated by construction).
- RemoveLandblock: fires OnRemove(entity.Id) for each ServerGuid==0
  entity before the loaded record is dropped.
- RemoveEntitiesFromLandblock: fires OnRemove for the demote-tier
  entities about to be cleared (Near→Far demotion).

5 new integration tests cover the four fire-sites + the no-double-fire
invariant on pending-bucket merges. Pattern matches existing
GpuWorldStateTests (stub LandBlock heightmap + WorldEntity factory).

Closes #56 end-to-end. Slice A (per-part transforms in Tasks 1-3) +
Slice B (dat-hydrated entity DefaultScript firing, this task) both
ready for visual verification at Holtburg portal + Inn fireplace +
cottage chimney + spell cast.

Note: 8 pre-existing failures in Physics/Input/MotionInterpreter test
families are unrelated to this work (verified by re-running with this
task's changes stashed).

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

src/AcDream.App/Streaming/GpuWorldState.cs

@@ -180,6 +180,21 @@ public sealed class GpuWorldState
         _loaded[landblock.LandblockId] = landblock;
         if (_wbSpawnAdapter is not null)
             _wbSpawnAdapter.OnLandblockLoaded(_loaded[landblock.LandblockId]);
+
+        // C.1.5b: fire DefaultScript for dat-hydrated entities (ServerGuid==0).
+        // Live entities (ServerGuid!=0) already had OnCreate fired at
+        // AppendLiveEntity; the filter avoids double-firing pending-bucket merges.
+        if (_entityScriptActivator is not null)
+        {
+            var loadedEntities = _loaded[landblock.LandblockId].Entities;
+            for (int i = 0; i < loadedEntities.Count; i++)
+            {
+                var e = loadedEntities[i];
+                if (e.ServerGuid == 0)
+                    _entityScriptActivator.OnCreate(e);
+            }
+        }
+
         RebuildFlatView();
     }
 
@@ -245,6 +260,19 @@ public sealed class GpuWorldState
                     _persistentRescued.Add(entity);
                 }
             }
+
+            // C.1.5b: stop DefaultScript for each dat-hydrated entity in
+            // the landblock. Server-spawned entities are either being
+            // rescued (script continues at the new LB) or were OnRemove'd
+            // via RemoveEntityByServerGuid earlier; leave them alone here.
+            if (_entityScriptActivator is not null)
+            {
+                foreach (var entity in lb.Entities)
+                {
+                    if (entity.ServerGuid == 0)
+                        _entityScriptActivator.OnRemove(entity.Id);
+                }
+            }
         }
 
         _pendingByLandblock.Remove(landblockId);
@@ -408,6 +436,18 @@ public sealed class GpuWorldState
         // canonicalized). Null when the cache isn't wired (tests). Per spec §5.3 W3b.
         _onLandblockUnloaded?.Invoke(canonical);
 
+        // C.1.5b: stop DefaultScript for each dat-hydrated entity about to
+        // be dropped. Demote-tier entities are always atlas-tier (ServerGuid==0
+        // per this method's class doc-comment); the filter is belt-and-suspenders.
+        if (_entityScriptActivator is not null)
+        {
+            foreach (var entity in lb.Entities)
+            {
+                if (entity.ServerGuid == 0)
+                    _entityScriptActivator.OnRemove(entity.Id);
+            }
+        }
+
         _loaded[canonical] = new LoadedLandblock(lb.LandblockId, lb.Heightmap, System.Array.Empty<WorldEntity>());
         _pendingByLandblock.Remove(canonical);
         RebuildFlatView();
@@ -447,6 +487,17 @@ public sealed class GpuWorldState
         _loaded[canonical] = new LoadedLandblock(lb.LandblockId, lb.Heightmap, merged);
         if (_wbSpawnAdapter is not null)
             _wbSpawnAdapter.OnLandblockLoaded(_loaded[canonical]);
+
+        // C.1.5b: fire DefaultScript for each promoted dat-hydrated entity.
+        // All entities arriving via this path are atlas-tier by construction
+        // (the promotion path streams in dat-static scenery + EnvCell statics
+        // + stabs per the method's class doc-comment).
+        if (_entityScriptActivator is not null)
+        {
+            for (int i = 0; i < entities.Count; i++)
+                _entityScriptActivator.OnCreate(entities[i]);
+        }
+
         RebuildFlatView();
     }