fix: clear stale nmmo3 entity observations

[jokrasno]

Summary

• clear nmmo3 observation entity bytes 4-9 when the authoritative pids cell is empty
• add a native regression that moves a real Entity between cells and exercises the production allocate_mmo() / compute_all_obs() path
• document Linux and macOS build/run commands for the native regression

Fixes #629.

Verification

On pristine 4.0 (c5d3c637), the regression failed after the entity moved:

empty cell retained entity bytes: [2, 3, 4, 4, 1, 2]
exit code: 1

The identically rebuilt test exits 0 after this change. It uses the real MMO and Entity structures and statically checks the 1,707-byte observation layout; no encoder logic is copied into the test.

Additional checks:

• optimized full nmmo3.c desktop build: passed
• python -m compileall -q pufferlib: passed
• pytest tests/test_import_performance.py -q: 1 passed
• git diff --check: passed

Compatibility note

This intentionally changes empty-cell observations from stale last-seen entity data to truthful snapshot semantics. As discussed in #629, existing weights may have learned to use the residue as an implicit memory channel. This PR keeps the correctness fix narrow; if compatibility needs an opt-in flag or environment-version boundary, I am happy to adapt it to the maintainers' preferred policy.

The native harness follows the repository's documented-command convention but is not currently discovered by existing CI. Wiring it into CI remains a maintainer integration decision.