The flap R1 exposed is a cell-membership ping-pong: the find_cell_list containing-
cell pick (CellTransit.BuildCellSetAndPickContaining) iterated an UNORDERED HashSet
and returned the first interior cell whose BSP contains the sphere center, with no
preference for the current cell. Retail CObjCell::find_cell_list adds the current
cell at index 0 (add_cell, pc:308766) and iterates current-first with interior-wins-
break (pc:308791-308819) — you STAY in your current cell until the center genuinely
leaves it. acdream's HashSet dropped that ordering; once the candidate set churns at
a boundary the enumeration can surface a neighbour before the current cell → the
ping-pong. Restore the explicit, deterministic current-cell-first test (retail's
index-0 hysteresis). + a two-direction regression guard (current cell wins the
straddle).
Diagnosed from the existing [cell-transit] walk log (no new probing): room flips are
the pick non-determinism; stairs flips additionally show the foot Z oscillating
~0.2m/tick (a separate stairs-physics residual, #98 family, to verify after this).
The 2 DoorBugTrajectoryReplay failures are PRE-EXISTING (verified: they fail without
this change too) — 2 of the handoff's '3 door-collision apparatus / A6.P5'.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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