P0 Task 4. FindCellList resolves a sphere deep inside room 0171 -> 0171,
deep inside vestibule 0170 -> 0170, and re-picks 0170 from a stale 0171
seed (membership re-picks by containment, not the seed). Retail-faithful
by construction (candidate cells loaded from the real dats). The subtle
doorway-threshold pick is the trace-backed golden (Task 6).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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