The pre-commit fetch cost ~6s on the measured cold sync (it did real work
absorbing a foreign push; ~3s + a full TLS handshake even when the remote is
unchanged). Drop it: push onto the current tip first, and only when the remote
rejects the push non-fast-forward do we fetch, mixed-reset onto origin, replay
the note, and retry. The happy path is now a single handshake.
- stage_and_commit extracted (used on the first attempt and the replay).
- reconcile_onto_origin replaces fast_forward_before_commit + fetch_and_integrate.
Mixed reset keeps the just-saved note; the replay lands it on origin's tip.
- Single-writer semantics: a foreign push resolves last-writer-wins instead of
bailing on divergence, so the device never gets stuck. A remote-only added file
it doesn't have would be dropped by the replay — needs the real merge path
(increment B), doesn't arise from this device's own use.
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