Apply the write checkpoint after draining the CRUD queue

[birdmanmandbir]

What changed

• Treat DONE from the post-delete CRUD query as an empty queue.
• Apply the provided write checkpoint only in that case.
• Add an integration regression test that completes the final CRUD transaction with checkpoint 42.

Root cause

SELECT 1 FROM ps_crud LIMIT 1 returns ROW when a CRUD item remains and DONE when the queue is empty. Checking OK meant the supplied checkpoint could never be selected.

Validation

• cargo fmt --all -- --check
• cargo test -p powersync applies_checkpoint_after_draining_crud_queue
• cargo test -p powersync --all-features
• cargo clippy -p powersync --all-targets -- -D warnings

Fixes #34