feat(render): indoor render WORKS — terminating portal flood + every-cell seal + look-in FPS

Checkpoint of the unified retail-faithful indoor render. The two-week HANG/grey is fixed and the
interior seals (live-verified by the user). Commits the session render-rewrite foundation together
with the fixes that made it functional.

- HANG fix: PortalVisibilityBuilder.Build portal flood did not terminate (the faithful ProjectToClip
  near-side clip drifts per round, defeating the CellView dedup; the BFS had no bound after U.2a removed
  MaxReprocessPerCell). Fix = drift-tolerant snapped/canonical CellView.Add dedup (PortalView.cs) plus
  restored MaxReprocessPerCell=16 bounded re-enqueue (PortalVisibilityBuilder.cs). Re-enqueue is kept
  (load-bearing for late-slice propagation, Build_ViewGrowthAfterDoneCell_PropagatesNewSlicesToExit);
  only its count is capped. CellViewDedupTests added.
- Seal (DrawCells Task 2): RetailPViewRenderer.DrawEnvCellShells draws EVERY visible cell via
  IndoorDrawPlan.ShellPass (was gated on the ClipFrameAssembler slot filter, leaving slot-less cells grey).
- Look-in FPS: GameWindow exterior look-in candidates limited to the player landblock +-1 (was all ~81
  loaded LBs iterated every outdoor frame). No behaviour change (far cells were >48m, already culled).

Remaining dominant issue = the FLAP at transitions: viewer-cell metastability (render roots at the
camera-eye cell, which oscillates outdoor-indoor as the 3rd-person boom drifts across the doorway,
confirmed in render-sig). SEPARATE fix, NOT the DrawCells port. Full handoff + flap fix plan + tracked
follow-ups (#78 terrain, look-in-from-inside, look-in FPS, L-spotlight):
docs/research/2026-06-07-indoor-render-session-handoff.md.

Baselines: build 0 err; App.Tests 210/210; Core.Tests 1331 pass / 4 fail (pre-existing) / 1 skip.

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

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

@@ -0,0 +1,64 @@
+using System.Numerics;
+using AcDream.App.Rendering;
+using Xunit;
+
+namespace AcDream.App.Tests.Rendering;
+
+// Regression tests for the indoor-render HANG (2026-06-06): the portal-visibility flood
+// re-queues a cell whenever its CellView grows, so it only terminates when CellView.Add's
+// dedup catches a duplicate. Across BFS rounds the same region comes back float-drifted,
+// vertex-rotated, or with a ±1 vertex count; the old exact index-by-index SamePolygon
+// (eps 1e-4) missed all three, so the region grew forever -> CPU-spin hang in CellView.Add.
+// A drift-tolerant, rotation-invariant dedup makes the key space finite, so the flood
+// converges (and these duplicates collapse).
+public class CellViewDedupTests
+{
+    private static ViewPolygon Quad(float ox, float oy) => new(new[]
+    {
+        new Vector2(ox - 0.5f, oy - 0.5f), new Vector2(ox + 0.5f, oy - 0.5f),
+        new Vector2(ox + 0.5f, oy + 0.5f), new Vector2(ox - 0.5f, oy + 0.5f),
+    });
+
+    [Fact]
+    public void Add_DropsSubGridDriftDuplicate()
+    {
+        var v = new CellView();
+        Assert.True(v.Add(Quad(0f, 0f)));
+        // Same quad, every vertex nudged 3e-4 — beyond the old 1e-4 SamePolygon eps,
+        // within the 1e-3 dedup grid. This is the per-round float drift that caused the hang.
+        var drifted = new ViewPolygon(new[]
+        {
+            new Vector2(-0.5f + 3e-4f, -0.5f - 3e-4f), new Vector2(0.5f + 3e-4f, -0.5f - 3e-4f),
+            new Vector2(0.5f + 3e-4f, 0.5f - 3e-4f), new Vector2(-0.5f + 3e-4f, 0.5f - 3e-4f),
+        });
+        Assert.False(v.Add(drifted));
+        Assert.Single(v.Polygons);
+    }
+
+    [Fact]
+    public void Add_DropsRotatedStartDuplicate()
+    {
+        var v = new CellView();
+        Assert.True(v.Add(Quad(0f, 0f)));
+        // Same 4 corners (same CCW cycle), but emitted starting at the 2nd vertex — what
+        // Sutherland-Hodgman can do when the subject order differs across rounds.
+        var rotated = new ViewPolygon(new[]
+        {
+            new Vector2(0.5f, -0.5f), new Vector2(0.5f, 0.5f),
+            new Vector2(-0.5f, 0.5f), new Vector2(-0.5f, -0.5f),
+        });
+        Assert.False(v.Add(rotated));
+        Assert.Single(v.Polygons);
+    }
+
+    [Fact]
+    public void Add_KeepsGenuinelyDistinctPolygons()
+    {
+        // The fix must NOT over-merge: two regions 0.4 NDC apart (far beyond the 1e-3 grid)
+        // remain distinct, so a real second portal opening is not silently dropped.
+        var v = new CellView();
+        Assert.True(v.Add(Quad(0f, 0f)));
+        Assert.True(v.Add(Quad(0.4f, 0f)));
+        Assert.Equal(2, v.Polygons.Count);
+    }
+}