[claude] Roll snapshots back to a checkpoint when a commit arrives out of order

[myieye]

[Claude, autonomous]

Fixes #105. The first commit adds the repro tests (LateCommitTests, one theory case of each fails on main), the second the fix.

• A late commit deleted the snapshots after it and replayed from each entity's newest surviving snapshot. That snapshot can predate edits whose snapshots a sync batch pruned, so the replay lost them and revived cascade-deleted entities (the FK failure in production).
• New local-only Commit.IsSnapshotCheckpoint ([JsonIgnore], like Hash): the last commit of every snapshot update is a checkpoint, meaning the snapshot table is complete as of that commit. A late commit rolls back to the newest checkpoint before it and replays everything after it; commits replayed inside that batch lose the flag. SnapshotCheckpointTests pins that contract for Refactor snapshot selection #34/Implement snapshot cleanup #35/Refactor snapshot selection #36.
• Schema change: one non-nullable bool column on Commits, consumers need a migration (EF scaffolds defaultValue: false).
• Existing databases have no checkpoints, so their first late commit replays from scratch (also heals snapshots the old rollback corrupted). Appends never roll back, so a healthy legacy DB accumulates checkpoints without ever paying that. An already-corrupted DB (FwHeadless) should run RegenerateSnapshots once, since flagging its head would preserve the corruption.
• RegenerateSnapshots now runs under the lock and in a transaction, so flags and snapshots move together.
• Not in this PR: GetAtCommit/GetBeforeCommit/GetSnapshotsAtCommit seed a replay the same way and have the same staleness (plus a double-apply of other entities' changes); they can use the checkpoint later. Making the sampler keep each entity's state at checkpoints inside a batch would make hub rollbacks cheaper and fits PR Refactor snapshot selection #34's post-pass.

🤖 Generated with Claude Code

https://claude.ai/code/session_017BaNW6P4wKvx82fgvh7q1j

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[myieye]

Superceded by #107