Pure-function aggregator that, given a CellPhysics.Resolved dict and
a foot local position, picks the nearest walkable-eligible polygon
(normal Z >= FloorZ) and reports XY-containment + signed vertical gap.
Also enumerates walkable polys with local-XY bboxes for the one-shot
[floor-polys] cell-load dump.
Pure-function, no behavior change. Wiring to emission sites lands in
the next commit.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
