Locked-database test mocks one query; add a test that holds a real lock #129

Found by the code-reviewer agent on #123.

test_locked_database_on_pending_check_warns makes the unresolved_refs query alone raise database is locked, so every other query in check() succeeds. With a real lock (BEGIN EXCLUSIVE from another process, journal_mode=DELETE), the pending query warns. Then stale_by_mtime (integrity.py:29) dies with an uncaught OperationalError after about 20 s of busy timeouts, exit 1.

That fails loud, so it is not a false green. But the test pins a path that a real lock never takes.

Fix: add a test that holds a real exclusive lock and asserts what testgraph actually does: a clean BLOCKED or retry message, and no traceback. The behaviour this test ends up asserting depends on the decision in the "database is locked" export-warning issue.