fix(session): make Save As sidecar cleanup transactional

[devmobasa]

Summary

• quarantine stale non-lock sidecars before replacing a Save As target
• restore quarantined recovery data when primary replacement fails
• clean quarantine contents only after the new primary is committed and synced
• retain recovery bytes in a private quarantine if post-commit cleanup fails
• cover canonical backup, recovery, marker, clear, and preserved-version artifacts while retaining the active lock

Validation

• transactional Save As failure-path tests
• runtime Save As behavior tests
• both focused suites with --no-default-features
• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• git diff --check