fix(render): Phase U.2b — resolve reciprocal portal by other_portal_id (retail 433557)

Code review caught a CRITICAL under-inclusion: ApplyReciprocalClip scanned for the
first OtherCellId match, so a cell with two portals to the same neighbour clipped both
near-side openings against the FIRST reciprocal polygon — hiding geometry through the
second opening (real on Holtburg cellar cells 0x148<->0x149). Plumb the dat's
OtherPortalId back-link through CellPortalInfo + BuildLoadedCell and index the reciprocal
directly (retail arg2->other_portal_id, 433557). Skip (degrade to over-include) when the
index is unresolvable — never clip against a guessed polygon. Adds a disjoint two-back-
portal regression test.

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

src/AcDream.App/Rendering/GameWindow.cs

@@ -5594,7 +5594,8 @@ public sealed class GameWindow : IDisposable
             portals.Add(new CellPortalInfo(
                 portal.OtherCellId,
                 portal.PolygonId,
-                (ushort)portal.Flags));
+                (ushort)portal.Flags,
+                portal.OtherPortalId));   // Phase U.2b: dat back-link → reciprocal portal index (retail 433557)
 
             // Build clip plane from the portal polygon.
             if (cellStruct.Polygons.TryGetValue(portal.PolygonId, out var poly)