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>

docs/ISSUES.md

@@ -146,21 +146,33 @@ Retail anchors (`docs/research/named-retail/acclient_2013_pseudo_c.txt`):
 - `InitCell` — per-cell timestamp init
 - `AddViewToPortals` 433446 — change-detection that drives the fixpoint
 
-**Related M-4 stub — CLOSED (Phase U.2b, 2026-05-30):** the
-neighbour-side `OtherPortalClip` (decomp:433524) is now ported. After a
-portal's near-side opening is clipped against the current cell's view,
-`PortalVisibilityBuilder.ApplyReciprocalClip` resolves the neighbour's
-matching back-portal (scan `neighbour.Portals` for the entry whose
-`OtherCellId` == the near cell's low-16-bits id; `PortalPolygons` is in
-lockstep), projects it through the neighbour's `WorldTransform`, and
+**Related M-4 stub — CLOSED (Phase U.2b, 2026-05-30; reciprocal-resolution
+fix 2026-05-30):** the neighbour-side `OtherPortalClip` (decomp:433524) is
+ported. After a portal's near-side opening is clipped against the current
+cell's view, `PortalVisibilityBuilder.ApplyReciprocalClip` resolves the
+neighbour's matching back-portal **by direct index via the dat's
+`CellPortal.OtherPortalId` back-link** (retail `arg2->other_portal_id`,
+005a54b2), projects it through the neighbour's `WorldTransform`, and
 intersects it into the propagated region before the union — so a cell's
-clip region is the intersection of the opening seen from BOTH sides. Can
-only TIGHTEN; degrades to prior near-side-only behavior when no
-back-portal is found. The `TODO(A8.F)` marker is removed. Covered by
+clip region is the intersection of the opening seen from BOTH sides. The
+reciprocal is `neighbour.PortalPolygons[portal.OtherPortalId]`, NOT a scan
+for the first `OtherCellId` match. The direct index is load-bearing: a cell
+with TWO portals to the same neighbour (real on the Holtburg cellar —
+`0x148` has two portals to `0x149`, polys 40/41, and `0x149` has two
+reciprocals back to `0x148`) clips each opening against its OWN reciprocal.
+The earlier scan-by-first-match resolved both near-side openings to the
+FIRST reciprocal, and disjoint apertures then intersected to empty —
+HIDING the geometry through the second opening (under-inclusion). The fix
+plumbs `OtherPortalId` through `CellPortalInfo` + `BuildLoadedCell`. Guards
+degrade to over-include (never clip against a guessed polygon) when the
+index is out of range, the polygon is missing/degenerate, or it projects
+behind the camera. Can only TIGHTEN. Covered by
 `PortalVisibilityBuilderTests.Build_AppliesReciprocalOtherPortalClip`
-(reciprocal tightening) + `…_DegradesGracefully_WhenNoBackPortal`.
-(The diamond-topology onward re-propagation of late growth remains out of
-scope here — tracked under U.6.)
+(reciprocal tightening) + `…_DegradesGracefully_WhenNoBackPortal`
+(over-include degrade) + `…_MultiplePortalsToSameNeighbour_EachResolvesOwnReciprocal`
+(the disjoint two-back-portal regression). (The diamond-topology onward
+re-propagation of late growth remains out of scope here — tracked under
+U.6.)
 
 **Files:**
 - `src/AcDream.App/Rendering/PortalVisibilityBuilder.cs` — replace the