After the primary cell's BSP returns OK, query every other cell the
foot-sphere overlaps via CellTransit.FindCellSet + Transition.CheckOtherCells.
Closes the Holtburg inn vestibule wall walk-through: the vestibule
(cell 0xA9B40164) has only 4 BSP polys; walls live in the adjacent
interior cell (0xA9B40157). Without A4 the adjacent cell's BSP was
never queried.
End-to-end test reduces the real Holtburg bug to a minimal synthetic
two-cell fixture: empty vestibule BSP + interior cell with the
existing BSPStepUpFixtures.TallWall (the same fixture B2 uses to
prove a grounded mover can't scale a 5m wall). Pre-A4: returns OK
(walks through). Post-A4: returns Slid (the wall halts the
transition).
FindEnvCollisions visibility tightened from private → internal so
the integration test can call it directly without going through
FindTransitionalPosition's sub-step iteration.
Retail oracle: acclient_2013_pseudo_c.txt:272717-272798
(CTransition::check_other_cells).
Spec: docs/superpowers/specs/2026-05-20-phase-a4-multi-cell-bsp-design.md
Plan: docs/superpowers/plans/2026-05-20-phase-a4-multi-cell-bsp.md
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
