Final mask check before publish leaves a narrow window before the manifest replace #222

This FILE item comes from the external review of #202 (ChatGPT batch 28, claim E5). It is not a blocker.

#202 re-checks every mask fingerprint, and SHA-256 in strict runs, in before_commit. That runs immediately before os.replace of run_manifest.json (run/generation.py:_commit). The output lock does not lock the source masks, so a mask can still change after the check returns and before or during the replace. The window is small, but a published manifest can bind bytes that are no longer on disk.

Decide which contract applies:

• If the contract is "check as late as practicable and detect ordinary concurrent edits", Re-check every mask fingerprint right before the manifest replacement #202 meets it. Document that wording in the contract and close this issue.
• If the contract is "the visible manifest always binds exactly the bytes present at commit", only snapshotting or locking the source masks can meet it, for example by binding the manifest to an immutable copy. Another check would not help.