feat(migrations): add atomic checkpoint storage operator

[seonghobae]

Linearized replacement for #63

This Draft replays the exact atomic checkpoint-migration operator delta onto replacement audit predecessor #79 exact head e99c99b081cd5d85f67faf1a0f46944110efbef3 without force-pushing or conflict-side selection.

RCA and replacement boundary

Old #63 remained based on superseded audit history after #62 was replaced by #79. Every file modified by #63 that overlaps its predecessor was verified byte-identical between old base ba2c102476f834c2c25750de8ede146e391d4c74 and replacement predecessor e99c99b081cd5d85f67faf1a0f46944110efbef3; no manual conflict-side selection was required.

Replacement head 3cac073cc791b78fafa0005b820e0206fe168a63 is exactly one commit ahead and zero behind exact predecessor #79 and changes exactly the same 21 migration-operator files as old #63: bounded canonical migration reads, ordered transaction-level advisory locking, all-or-nothing 0007/0008 application, canonical success evidence, CLI/public API, exact-source CI verification, live PostgreSQL rollback/concurrency tests, and authoritative docs.

Old #63 is closed unmerged as superseded. No predecessor-head check, review, or approval transfers.

Current staged evidence

• Head: 3cac073cc791b78fafa0005b820e0206fe168a63.
• Exact current base tip: e99c99b081cd5d85f67faf1a0f46944110efbef3 (feat(audit): persist append-only checkpoint acceptance trail #79).
• Ancestry: one commit ahead, zero behind; merge base equals the exact base tip.
• CI 31286065261: completed / success on the exact replacement source head.
• Release Acceptance 31286065264: completed / success on the exact replacement source head.
• Formal reviews: none at the latest inspection.
• Unresolved inline review threads: zero.

This is current stacked-stage evidence only. Default-branch-only organization/security/review gates are not inferred as passing while this PR is stacked.

Merge boundary

Required order remains .github#790 -> pg-llm-batch#53 -> #55 -> #56 -> #57 -> #58 -> #59 -> #60 -> #78 -> #79 -> this PR -> #83.

Keep Draft while earlier prerequisites remain unintegrated. After every prerequisite safely reaches protected main, reconcile or retarget this slice onto the actual integrated base and regenerate all required exact-head/exact-base CI, live PostgreSQL migration/rollback/concurrency, security, dependency, SBOM/provenance, packaging, automated-review, branch-protection, repository-policy, and Release Acceptance gates. Merge only with zero unresolved valid findings and a qualifying independent non-author GitHub APPROVED review on the unchanged final head. Queued, pending, failed, cancelled, skipped-required, neutral-required, absent, stale-head/base, predecessor, status-only, author-only, or synthetic-only evidence is never acceptance.

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[seonghobae]

Superseded by #95. Fresh comparison proves #95 descends from current audit replacement #94 at exact head cc7c862cff76521ead8696c7d6bfe1a2c4b31780 and reproduces exactly the same 21-file migration-operator delta and aggregate 2,045 additions / 19 deletions as this PR, while preserving the newer checkpoint-DSN CHANGELOG boundary. Historical checks/reviews/approvals do not transfer; #95 must reacquire its own exact-head/exact-base evidence.