[Recovery] Point-in-Time Snapshot and Copy-on-Write Restore

[avelino]

ChronDB’s Backup/Restore system enables full recovery, but lacks atomic point-in-time snapshots for branches.
This feature should allow "freezing" a consistent view of data and restoring it later without mutating history.

Why
Snapshots simplify rollback, auditing, and experimentation (e.g., sandboxing before migrations).

What to do

• Add commands:

snapshot create --branch main --tag release-2025
restore --snapshot release-2025

• Snapshots should be stored as Git tags or branches.
• Restore creates a new commit (copy-on-write), not a rewrite.

Acceptance Criteria

• Snapshot creation preserves state atomically.
• Restore produces new commit maintaining lineage.
• Documented with usage examples.

References

• Git tags and snapshots: https://git-scm.com/book/en/v2/Git-Basics-Tagging
• Post about restoration model (ChronDB design): internal doc