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

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+// 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;
+    }
+}

tests/AcDream.App.Tests/Rendering/IndoorDrawPlanTests.cs

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+using System.Collections.Generic;
+using System.Linq;
+using System.Numerics;
+using AcDream.App.Rendering;
+using Xunit;
+
+namespace AcDream.App.Tests.Rendering;
+
+public class IndoorDrawPlanTests
+{
+    private static ViewPolygon Quad() => new(new[]
+    { new Vector2(-1, -1), new Vector2(1, -1), new Vector2(1, 1), new Vector2(-1, 1) });
+
+    private static PortalVisibilityFrame FrameWith(params uint[] orderedCells)
+    {
+        var f = new PortalVisibilityFrame();
+        foreach (var id in orderedCells)
+        {
+            f.OrderedVisibleCells.Add(id);
+            var v = new CellView(); v.Add(Quad());
+            f.CellViews[id] = v;
+        }
+        return f;
+    }
+
+    [Fact]
+    public void ShellPass_IncludesEveryVisibleCell_NoFilter()
+    {
+        // The grey bug: a cell in OrderedVisibleCells must NEVER be dropped from the
+        // shell pass. (Old code dropped cells lacking a ClipFrameAssembler slot.)
+        var f = FrameWith(0x01, 0x02, 0x03);
+        var plan = IndoorDrawPlan.ShellPass(f);
+        Assert.Equal(new uint[] { 0x03, 0x02, 0x01 }, plan.Select(e => e.CellId).ToArray()); // reverse = far→near
+        Assert.All(plan, e => Assert.NotEmpty(e.Slices));
+    }
+
+    [Fact]
+    public void ShellPass_ExcludesEmptyViewCells()
+    {
+        var f = FrameWith(0x01);
+        f.OrderedVisibleCells.Add(0x02);     // present in the list…
+        f.CellViews[0x02] = new CellView();  // …but empty view → not drawable
+        var plan = IndoorDrawPlan.ShellPass(f);
+        Assert.Equal(new uint[] { 0x01 }, plan.Select(e => e.CellId).ToArray());
+    }
+}