Add private `SetCurrAndReturn(uint)` helper in PhysicsEngine that looks up
the resolved id in `DataCache.CellGraph` and writes `CurrCell` when the cell
is present. Wrap the four RESOLVED-id return sites in ResolveCellId:
- indoor no-CellBSP return (trust FindCellList)
- indoor sphere-overlaps-CellBSP return
- outdoor→indoor building-transit return (foreach candidate)
- outdoor terrain-grid return
The final no-match `return fallbackCellId;` is intentionally NOT wrapped —
stale beats null (the caller's seed is preserved unchanged).
CurrCell has zero readers in src/ (verified by ripgrep); this is additive
write-only, identical observable behavior to W1. One new unit test
(CellGraphMembershipTests) proves RED→GREEN.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
