"the merge queue > a gate that never answers is abandoned at the deadline, and the next sweep lands (#3430)" passes every time on a loaded check machine #5611

Done:

• [ ] bun test --rerun-each 30 test/merge-queue.test.ts -t "a gate that never answers is abandoned at the deadline" passes, and the pull names the cause it found.
• [ ] Nothing the test promises is weakened: no longer timeouts, no retries, no dropped assertion. Drive the deadline with an injected clock, or wait on the sweep's event, never a sleep.
• [ ] If the cause is in the product (the deadline measured on a wall clock under load, say), fix the product and say so.

Files: test/merge-queue.test.ts and the module the cause is in. Why now: it sank both tries of #5395 (pulls #5597 and #5604), which never touched the merge queue.