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>

tests/AcDream.App.Tests/Rendering/Wb/BuildingLoaderTests.cs

@@ -136,7 +136,7 @@ public class BuildingLoaderTests
             CellId = 0xA9B40150u,
             Portals = new List<AcDream.App.Rendering.CellPortalInfo>
             {
-                new(0xFFFF, 0, 0),
+                new(0xFFFF, 0, 0, 0),
             },
             PortalPolygons = new List<Vector3[]>
             {