perf(pipeline): MP-Alloc Task 4 - reuse per-frame animatedIds + drawableCells sets

Completes the safe-batch (implementer finished this but died before
committing; coordinator independently verified bit-identity + safety
post-interruption). Two per-frame HashSet<uint> allocations replaced by
reused cleared-in-place fields:
- GameWindow._animatedIdsScratch: WalkEntitiesInto treats null and empty
  identically (line 682 `is null || Count == 0`), so an always-non-null
  empty set == the old null-when-empty local. Verified.
- RetailPViewRenderer._drawableCellsScratch: flows into
  RetailPViewFrameResult.DrawableCells (live ref) but every consumer reads
  it same-frame; sigDrawableCells is a per-frame OnRender local that
  EmitRenderSignatureIfChanged only FORMATS to a string (no reference
  retention, no _lastSig set field). Built frame N -> read frame N ->
  cleared frame N+1. No cross-frame corruption.

Build green, full suite 4116 passed / 4 skipped.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/AcDream.App/Rendering/GameWindow.cs

@@ -252,6 +252,13 @@ public sealed class GameWindow : IDisposable
     /// </summary>
     private readonly Dictionary<uint, AnimatedEntity> _animatedEntities = new();
 
+    // MP-Alloc (2026-07-05): reusable per-frame snapshot of _animatedEntities.Keys.
+    // WbDrawDispatcher.WalkEntitiesInto treats null and an empty set identically
+    // (`animatedEntityIds is null || animatedEntityIds.Count == 0`), so this can
+    // always be a live (possibly empty) HashSet instead of `new`ing one every
+    // frame or passing null.
+    private readonly HashSet<uint> _animatedIdsScratch = new();
+
     /// <summary>
     /// Tier 1 cache (#53): per-entity classification results for static
     /// entities (those NOT in <see cref="_animatedEntities"/>). Conceptually
@@ -9104,13 +9111,15 @@ public sealed class GameWindow : IDisposable
             var envCellViewProj = camera.View * camera.Projection;
             _envCellFrustum?.Update(envCellViewProj);
 
-            HashSet<uint>? animatedIds = null;
+            // MP-Alloc: reuse _animatedIdsScratch instead of `new`ing a
-            if (_animatedEntities.Count > 0)
+            // HashSet<uint> every frame. Downstream consumers (WbDrawDispatcher.
-            {
+            // WalkEntitiesInto) treat null and an empty set identically, so an
-                animatedIds = new HashSet<uint>(_animatedEntities.Count);
+            // always-non-null (possibly empty) set is behaviorally the same as
-                foreach (var k in _animatedEntities.Keys)
+            // the old null-when-nothing-animated local.
-                    animatedIds.Add(k);
+            _animatedIdsScratch.Clear();
-            }
+            foreach (var k in _animatedEntities.Keys)
+                _animatedIdsScratch.Add(k);
+            HashSet<uint>? animatedIds = _animatedIdsScratch;
 
             // Phase G.1: sky renderer — draws the far-plane-infinity
             // celestial meshes FIRST so the rest of the scene z-tests

src/AcDream.App/Rendering/RetailPViewRenderer.cs

@@ -50,6 +50,13 @@ public sealed class RetailPViewRenderer
     // visible.
     private readonly InteriorEntityPartition.Result _partitionResult = new();
 
+    // MP-Alloc (2026-07-05): DrawInside's drawable-cell set, reused across
+    // frames instead of `new HashSet<uint>(pvFrame.OrderedVisibleCells)` every
+    // call. Every consumer (DrawEntityBucket, DrawExitPortalMasks,
+    // DrawCellObjectLists, RetailPViewFrameResult.DrawableCells) reads it
+    // synchronously within the same frame it was built.
+    private readonly HashSet<uint> _drawableCellsScratch = new();
+
     // T2 (BR-4): retail has NO distance constant on the flood-admission chain
     // (DrawBuilding → portal walk → ConstructView: viewconeCheck + side test +
     // GetClip + GetVisible only). The old 48 m seed cap is replaced by the
@@ -118,7 +125,9 @@ public sealed class RetailPViewRenderer
         // so every visible cell's shell has a prepared batch and seals — killing the grey
         // (the old clipAssembly.CellIdToSlot.Keys filter silently dropped slot-less cells).
         // Per-slice trim still applies in DrawEnvCellShells (Task 4 makes it self-contained).
-        var drawableCells = new HashSet<uint>(pvFrame.OrderedVisibleCells);
+        _drawableCellsScratch.Clear();
+        _drawableCellsScratch.UnionWith(pvFrame.OrderedVisibleCells);
+        var drawableCells = _drawableCellsScratch;
         UseIndoorMembershipOnlyRouting();
 
         // #124: look-in cells need prepared shell batches + their statics routed