[AGiorgetti]

Summary

Fixes #90.

• Advance SnapshotRevision atomically and subtract only its advance from Unsnapshotted. Preserve concurrent and delayed commit increments and configured write concerns.
• Preserve missing-head validation and use a documented fallback for legacy missing/null counters. No historical counter repair or lifecycle redesign is claimed.
• Add deterministic sync/async regression coverage. The CI workflow is restored byte-for-byte to the PR base; temporary integration-only test setup is removed.

Breaking change

MongoDB 4.2+ is required for update pipelines. Upgrade the server before the adapter. No stored-schema migration is needed. Version 4.2 is a feature minimum, not a recommended production version. README and changelog document this requirement.

Validation

Rebased onto develop at 88fdb6b9228b0ad56642b04bab81f9a203349951, including the merged #88 and #89 fixes. The changelog conflict is resolved, and the CI workflow remains identical to the PR base.

Final head fa6f1ecc398ea3086a2b5ddc64f51011b1d53da4: exact-head standard CI passed: Windows and Ubuntu builds plus MongoDB 9 test lanes for net8/net9/net10, each 579/579 passed with zero failures and zero skips.

Historical run 41542168410 at 8155b08:

• MongoDB 9: 512/512 passed on each of net8/net9/net10, with zero skips.
• One-time combined runtime verification with PR Fix regressing stream-head revisions after reordered writes (#89) #102 pinned at d12d48a: 536/536 passed, including actual-engine delayed metadata/snapshot interleavings, with zero skips. That temporary job and fixture are not retained.
• MongoDB 4.2.24: 511/512 passed, zero skips. The existing Stream_head_delete_by_id_should_use_the_builtin_id_index explain-plan assertion failed; this is not a full minimum-version suite pass. No assertion was weakened, fixed, or excluded. The temporary 4.2 job is removed at the user's request.

No local build/tests were run. CI resolves Driver 3.9, not the library's declared 3.0 minimum. MongoDB 4.2 remains the documented feature minimum, not a claim of complete green minimum-version coverage.

Co-authored by Alessandro Giorgetti and Codex (OpenAI).