Atomic commit via put-if-absent

So far one writer bumps the version. What makes two writers safe? A commit must be atomic: either v<N+1>.json becomes visible in full, or not at all, and two writers must never both believe they created the same version.

Object stores give you exactly one tool for this: a conditional create — "write this key only if it does not already exist". objectstore.py exposes it as put_if_absent(key, data) -> bool (True = you created it). On S3 this is the If-None-Match: * conditional PUT; here it's O_CREAT|O_EXCL. You build the commit protocol on it; you never re-implement the primitive.

Implement (refactor your commit path)

Make every mutation (append, and later delete/compact) go through one commit routine:

1. Read the latest version N and its metadata (the base).
2. Build the new snapshot N+1 from the base.
3. ok = store.put_if_absent("<name>/metadata/v{N+1}.json", data).
4. If ok → committed, return the new snapshot id. If not ok → someone else committed N+1 first. Go back to step 1 and retry against the new latest (this is the stage-4 retry loop; wire it in now).

What the tests check

• A commit is all-or-nothing: no half-written metadata is ever observable.
• You use put_if_absent for the version object (a plain overwrite would let two writers clobber each other — that's a lost update, tested in stage 4).

Keep stages 1–2 green.