Clip the portal opening against the neighbour's matching back-portal polygon
before propagating, so a cell's clip region is the intersection of the opening
seen from both sides. Closes the M-4 stub in ISSUES #102. Can only tighten,
never under-include; degrades to prior behavior when no back-portal is found.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
PortalVisibilityFrame gains OrderedVisibleCells (closest-first). Replace the FIFO +
MaxReprocessPerCell cap with a distance-priority queue and a grow-watermark fixpoint
(retail InsCellTodoList 433183 / AddViewToPortals 433446) so cyclic dungeon graphs
converge without duplicate-cell blow-up.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
