T6 (BR-7) C1: signed OtherPortalId + the >=0 building-transit gate

Retail CEnvCell::check_building_transit (Ghidra 0x0052c5d0) opens with
`if (other_portal_id >= 0)` on the SIGNED sign-extended portal id
(CBldPortal.other_portal_id is int, acclient.h:32098). Our BldPortalInfo
carried the dat reader's raw ushort and CheckBuildingTransit had no gate
at all, so a portal whose dat value is 0xFFFF (-1, "no reciprocal
portal") could admit its interior cell. BN's pseudo-C renders the
comparison unsigned - the sign-extension is Ghidra-proven (BR-7 verified
corrections, wf1-interior-collision.md).

- BldPortalInfo.OtherPortalId: ushort -> short; GameWindow construction
  reinterprets the dat ushort via unchecked((short)).
- CheckBuildingTransit: negative-id portals rejected before any sphere
  test; new multi-sphere overload matching retail's per-sphere loop
  (0052c5fe, first-hit admits) with the hits_interior_cell output
  (0052c650) the BR-7 building channel consumes next.
- Tests: negative-id skip vs positive-id admit on a leaf-root CellBSP;
  multi-sphere plumbing + zero-sphere no-op.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/AcDream.App/Rendering/GameWindow.cs

@@ -5967,7 +5967,12 @@ public sealed class GameWindow : IDisposable
                     {
                         bldPortals.Add(new AcDream.Core.Physics.BldPortalInfo(
                             otherCellId:   lbPrefix | (uint)bp.OtherCellId,
-                            otherPortalId: bp.OtherPortalId,
+                            // DatReaderWriter parses the dat's 16-bit field as
+                            // ushort; retail sign-extends it to a SIGNED int
+                            // (CBldPortal.other_portal_id, acclient.h:32098) and
+                            // check_building_transit gates on `>= 0`
+                            // (Ghidra 0x0052c5dc). 0xFFFF → -1 = no reciprocal.
+                            otherPortalId: unchecked((short)bp.OtherPortalId),
                             flags:         (ushort)bp.Flags));
                     }