Add atomic managed rollback for database replacement

[Widthdom]

Summary

• Create a verified managed rollback snapshot before import, checkpoint restore, or restore-by-ID replaces an existing database, unless the caller explicitly passes --no-backup.
• Add cdidx db restore-backups --restore <id> with non-mutating dry-run, bounded manifest parsing, SHA-256 and schema validation, free-space checks, atomic replacement, and rollback preservation on failure.
• Keep restore-backup list/prune compatibility while exposing managed IDs and path-free provenance metadata.
• Cover missing destinations, corrupt payloads/imports, insufficient space, supported older schemas, cancellation, rollback failures, filesystem case semantics, Windows locking, and pooled SQLite handles before Windows replacement.

Validation

• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• dotnet build CodeIndex.sln -c Debug --no-restore (0 warnings, 0 errors)
• dotnet build CodeIndex.sln -c Release --no-restore (0 warnings, 0 errors)
• After the latest origin/main sync, Release Issue4857ManagedRestoreBackupTests and ConsoleUiTests: 292/292 passed on both net8.0 and net9.0
• Earlier Debug merge-sensitive tests: 379/379 passed on both net8.0 and net9.0
• Earlier Release Issue Provide atomic rollback for database import and pre-restore backups #4857 / restore tests: 35/35 passed on both net8.0 and net9.0
• Earlier full Release suite:
  • net9.0: 10,251 passed, 419 skipped, 0 failed
  • net8.0: 10,723 passed, 7 skipped; one unrelated watcher timing test failed during the full run and then passed 1/1 on isolated retry
• dotnet run --project tools/CodeIndex.Changelog -- check (38 fragments validated)
• cdidx status --check --json (index_matches_workspace: true, no failed checks)
• Two Codex adversarial review passes completed; all actionable findings were addressed and regression-tested.

Documentation and changelog

• Updated the English and Japanese sections of USER_GUIDE.md, DEVELOPER_GUIDE.md, and TESTING_GUIDE.md.
• Added changelog.d/unreleased/4857.fixed.md, with English/Japanese release notes and references to the prior rollback/retention issues.

Follow-up candidates

• None.

Fixes #4857