test: add git-branch divergence matrix + replay fidelity guards (D1, D4, D5)

Systematize the local-v1 divergence & recovery matrix (the most re-broken area, #953/#1251/#1252/#1260) plus the specifically-untested replay edges.

D1 (integration, git-branch): drive the local-v1 state {ahead, behind, diverged, disconnected} through the real triggers end-to-end:

• pre-push (real hook): local-only survives; ahead fast-forwards; diverged replays both sides on a linear tip; disconnected cherry-picks; a repeated push is idempotent (double-replay guard, #1260).
• explain fetch-on-miss for {behind, diverged}, extending the existing {missing, ahead} coverage: a remote-only checkpoint is fetched/reconciled and local-only checkpoints survive.
• doctor --force reconciles a disconnected v1, preserving both sides.

D4 (strategy unit): replay fidelity edges. A no-op commit must not clobber the remote tip's accumulated tree (regression 743c43f4c); a root-commit chain replays cleanly. Plus a KNOWN BUG pin: >1000-commit replay is still capped (collectCommitsSince) because the cap-removal (4cf01edb3) lives on the unmerged branch origin/no-limit; flip the assertion when it lands.

D5 (strategy): TOCTOU guard for 1e8628ade -- fetchAndRebaseRefCommon replays onto the freshly fetched remote tip, not a stale earlier hash. The literal ls-remote seam the fix patched no longer exists (fix is an ancestor), so this pins the behavioural invariant.