perf(pview): batch EnvCell shell opaque pass + skip empty transparent

Dense-town FPS root cause: DrawEnvCellShells called the heavy per-frame
EnvCellRenderer.Render once PER cell x opaque+transparent (~94 calls/frame,
24.75ms = 75% of GPU at Arwic). Batch opaque into one Render(Opaque, allCells)
(z-buffer order; per-instance CellId-keyed lighting => safe) + skip the
transparent Render for opaque-only cells, keep far->near for the rest.

Measured (Arwic, same facing, profiler on): cells 94 calls/24.75ms -> 1 call/
0.37ms; frame 34ms/29fps -> ~10ms/100fps p50.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/AcDream.App/Rendering/RetailPViewRenderer.cs

@@ -23,6 +23,11 @@ public sealed class RetailPViewRenderer
         new(0, new Vector4(-1f, -1f, 1f, 1f), Array.Empty<Vector4>());
 
     private readonly HashSet<uint> _oneCell = new(1);
+    // Shell-batch scratch: all of a pass's cells collected for ONE batched
+    // opaque Render call (instead of one heavy Render per cell). Reused across
+    // frames + across look-in buildings. Spec:
+    // docs/superpowers/specs/2026-06-23-envcell-shell-batching-design.md
+    private readonly HashSet<uint> _shellBatch = new();
 
     // R-A2: per-building flood grouping, reused across frames (inner lists cleared each frame).
     private readonly Dictionary<uint, List<LoadedCell>> _buildingGroups = new();
@@ -656,11 +661,28 @@ public sealed class RetailPViewRenderer
         // Per-cell opaque+transparent keeps the far→near transparent
         // compositing the per-cell loop already provided.
         UseIndoorMembershipOnlyRouting();
+
+        // Opaque: ONE batched Render for all shell cells (was one heavy per-frame
+        // Render call PER cell — the dense-town FPS sink, ~94 calls/24.75ms at
+        // Arwic). Opaque needs no draw order (z-buffer), and lighting is
+        // per-instance (CellId-keyed light SSBO in EnvCellRenderer.RenderModernMDI-
+        // Internal), so cross-cell batching is visually identical. The filtered
+        // Render path already groups all cells' instances into one MDI.
+        _shellBatch.Clear();
+        foreach (var entry in IndoorDrawPlan.ShellPass(pvFrame))
+            _shellBatch.Add(entry.CellId);
+        if (_shellBatch.Count > 0)
+            _envCells.Render(WbRenderPass.Opaque, _shellBatch);
+
+        // Transparent: far→near order matters for compositing, so keep these
+        // per-cell in ShellPass order — but skip cells with no transparent
+        // geometry (most are opaque-only walls/floors), removing the bulk of the
+        // per-cell transparent Render calls.
         foreach (var entry in IndoorDrawPlan.ShellPass(pvFrame))
         {
+            if (!_envCells.CellHasTransparent(entry.CellId)) continue;
             _oneCell.Clear();
             _oneCell.Add(entry.CellId);
-            _envCells.Render(WbRenderPass.Opaque, _oneCell);
             _envCells.Render(WbRenderPass.Transparent, _oneCell);
         }
     }