feat(render): IndoorDrawPlan.ShellPass — every visible cell, no drawable filter (R1)

Pure port of retail DrawCells membership: reverse cell_draw_list, per-slice. Pins the
grey regression — a cell in OrderedVisibleCells is never dropped from the shell pass.

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

src/AcDream.App/Rendering/IndoorDrawPlan.cs

@@ -0,0 +1,30 @@
+// IndoorDrawPlan.cs
+//
+// Pure (GL-free) port of the membership half of retail PView::DrawCells (0x5a4840):
+// the reverse cell_draw_list iterated per portal_view slice. EVERY visible cell with a
+// non-empty view is included — there is NO "drawable" filter. Dropping cells without a
+// clip-slot was the grey-walls bug (the cell's sealed shell never drew → clear color showed).
+using System.Collections.Generic;
+
+namespace AcDream.App.Rendering;
+
+public readonly record struct CellDrawEntry(uint CellId, IReadOnlyList<ViewPolygon> Slices);
+
+public static class IndoorDrawPlan
+{
+    /// <summary>Reverse OrderedVisibleCells (far→near), each visible cell with its view
+    /// slices. Mirrors DrawCells' shell/object loops. Cells whose view is empty are skipped
+    /// (they are not actually visible); no other cell is ever dropped.</summary>
+    public static List<CellDrawEntry> ShellPass(PortalVisibilityFrame frame)
+    {
+        var result = new List<CellDrawEntry>(frame.OrderedVisibleCells.Count);
+        for (int i = frame.OrderedVisibleCells.Count - 1; i >= 0; i--)
+        {
+            uint cellId = frame.OrderedVisibleCells[i];
+            if (!frame.CellViews.TryGetValue(cellId, out var view) || view.IsEmpty)
+                continue;
+            result.Add(new CellDrawEntry(cellId, view.Polygons));
+        }
+        return result;
+    }
+}