feat(checkpoint): add storage checkpoint API

[KKould]

What problem does this PR solve?

KiteSQL did not expose a storage-level checkpoint API, which made it harder to take online consistent snapshots from native embeddings.

Issue link:

What is changed and how it works?

• Add CheckpointableStorage and Database::checkpoint(path) so checkpoint support is exposed as an optional storage capability.
• Implement checkpoint support for RocksDB-backed storage.
  • build_optimistic() uses RocksDB's safe checkpoint path.
  • build_rocksdb() is gated behind unsafe_txdb_checkpoint because upstream rocksdb does not expose a safe TransactionDB checkpoint API.
• Add a small build.rs guard so unsafe_txdb_checkpoint only builds with the currently validated rocksdb and librocksdb-sys versions in Cargo.lock.
• Add coverage for disabled mode, non-empty target directories, restore-from-checkpoint, and checkpoint-under-concurrent-writes.
• Document checkpoint support in docs/features.md, keep the README summary short, and bump kite_sql to 0.2.1.

Code changes

• Has Rust code change
• Has CI related scripts change

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Note for reviewer

Validation used for this branch:

• cargo test checkpoint -- --nocapture
• cargo test --features unsafe_txdb_checkpoint checkpoint -- --nocapture