Add CellPosition (retail Position type) alongside the world Vector3 Position.
The Position setter mirrors each world delta into the cell-local origin and
calls AdjustToOutside only when the local coord crosses a landblock boundary
([0,192) on X or Y), so the within-block cell id is preserved from the wire
seed. SnapToCell seeds both positions from the wire's (cell, local) pair
verbatim — no streaming center involved. Unseeded bodies (ObjCellId==0) and
indoor cells are no-ops in the delta path. UpdatePhysicsInternal's existing
`Position +=` desugars through the new setter automatically; no call sites
changed. 4 new unit tests; full Core suite 1526 passed / 0 failed / 2 skipped.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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