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

First slice of the indoor-cell visibility culling pipeline (#78). Adds
PortalPolygons: List<Vector3[]> to LoadedCell, parallel-indexed to the
existing Portals + ClipPlanes lists. Empty arrays for portals whose
polygon could not be resolved. Field is populated in Task 2.

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

src/AcDream.App/Rendering/CellVisibility.cs

@@ -56,6 +56,20 @@ public sealed class LoadedCell
     /// Derived from portal polygon geometry during cell preparation.
     /// </summary>
     public List<PortalClipPlane> ClipPlanes = new();
+
+    /// <summary>
+    /// Portal polygon vertices in cell-local space, one Vector3[] per
+    /// <see cref="CellPortalInfo"/> entry in <see cref="Portals"/>. Index i
+    /// in this list corresponds to index i in <see cref="Portals"/> and
+    /// <see cref="ClipPlanes"/>. An empty array means the portal's polygon
+    /// could not be resolved at load time (degenerate cell or missing
+    /// polygon entry).
+    /// <para>
+    /// Used by the Phase A8 indoor-cell stencil pipeline to build a
+    /// per-frame triangle-fan mesh for portal silhouette masking.
+    /// </para>
+    /// </summary>
+    public List<Vector3[]> PortalPolygons = new();
 }
 
 /// <summary>