test: explicitly release stale claim lock

[tomatotomata]

Summary

• make the stale scheduled-claim restart test explicitly release its OS file lock before recovery
• preserve the existing explicit lease timestamps and all production recovery assertions
• keep the change test-only and scoped to the flaky regression

Closes #179

Validation

• cargo fmt --all --check passed
• git diff --check passed

The focused cargo test --locked command could not compile this Windows checkout because the repository currently imports Unix-only APIs (std::os::unix, tokio::signal::unix) in the default test build. The patch uses the same explicit lock._file.unlock() pattern already present in lock_and_ledger_prevent_overlap_and_recover_stale_claim.

AI-assisted contribution. I reviewed the test path and existing lock-release precedent manually.