feat(render): Phase A8 — populate LoadedCell.PortalPolygons

BuildLoadedCell now reads the full portal polygon vertices from
cellStruct.Polygons[portal.PolygonId].VertexIds and stores them in
local-space on the LoadedCell. Empty arrays for unresolved polygons.
Same source as the ClipPlane block; no new dat read.

Unit test covers the data-class invariant (parallel indexing) since
the full integration is exercised only at runtime with live dat data.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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

@@ -0,0 +1,55 @@
+// CellVisibilityPortalPolygonsTests.cs — verifies BuildLoadedCell preserves
+// portal polygon vertices in LoadedCell.PortalPolygons.
+//
+// Phase A8 — Indoor-cell visibility culling. Issue #78.
+
+using System.Numerics;
+using AcDream.App.Rendering;
+using Xunit;
+
+namespace AcDream.App.Tests.Rendering;
+
+public class CellVisibilityPortalPolygonsTests
+{
+    [Fact]
+    public void LoadedCell_DefaultPortalPolygons_IsEmpty()
+    {
+        var cell = new LoadedCell();
+        Assert.NotNull(cell.PortalPolygons);
+        Assert.Empty(cell.PortalPolygons);
+    }
+
+    [Fact]
+    public void LoadedCell_PortalPolygons_ParallelIndexedToPortals()
+    {
+        // Hand-construct: two portals, one with a 4-vertex polygon, one with
+        // an empty polygon (resolution failure simulated by an empty array).
+        // The data-class semantics: PortalPolygons[i] corresponds to
+        // Portals[i] and ClipPlanes[i].
+        var cell = new LoadedCell
+        {
+            Portals = new()
+            {
+                new CellPortalInfo(0xFFFF, 100, 0),  // exit portal, has geometry
+                new CellPortalInfo(0x0102, 101, 0),  // inner portal, no geometry resolved
+            },
+            ClipPlanes = new() { default, default },
+            PortalPolygons = new()
+            {
+                new[]
+                {
+                    new Vector3(0, 0, 0),
+                    new Vector3(1, 0, 0),
+                    new Vector3(1, 1, 0),
+                    new Vector3(0, 1, 0),
+                },
+                System.Array.Empty<Vector3>(),
+            },
+        };
+
+        Assert.Equal(2, cell.Portals.Count);
+        Assert.Equal(2, cell.PortalPolygons.Count);
+        Assert.Equal(4, cell.PortalPolygons[0].Length);
+        Assert.Empty(cell.PortalPolygons[1]);
+    }
+}