fix: clean up cancelled LockMap acquirers - #51

Fixes #33.

Tracks pending lock acquirers explicitly with an RAII handle, so cancellation between lookup and lock registration cannot leave an empty LockMap entry behind. Cleanup waits for every pending acquirer and active lock holder.

Regression coverage:

• cancelled_acquirer_removes_the_abandoned_entry
• cleanup_waits_for_every_acquirer_to_drop

Stacked on #48.

Validation:

• cargo fmt --check
• cargo clippy --all-targets -- -D warnings
• cargo test