Port of retail's CTransition::check_other_cells at
acclient_2013_pseudo_c.txt:272717-272798. Iterates every non-primary
cell in a candidate set, runs BSPQuery.FindCollisions per cell with
that cell's WorldTransform-derived rotation + origin, halts on first
Collided/Adjusted/Slid.
ApplyOtherCellResult is the combine-semantics helper extracted for
unit testability — it pins the retail switch:
- Collided/Adjusted → CollidedWithEnvironment = true (gated on
!Contact), halt.
- Slid → ContactPlaneValid + ContactPlaneIsWater = false,
halt.
- OK → continue.
Not yet wired into FindEnvCollisions — see next commit. Probe gated
on PhysicsDiagnostics.ProbeIndoorBspEnabled (ACDREAM_PROBE_INDOOR_BSP).
Six new unit tests: five against the pure combine helper for each halt
case + one direct CheckOtherCells call exercising the null-BSP guard.
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>
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