"RepoSource: one repository, two places to read it from > a mirror reports its stages in order and ends holding the repo" passes every time on a loaded check machine #5394

Done:

• [ ] bun test --rerun-each 30 test/view-source.test.ts -t "a mirror reports its stages in order" passes, and the pull names the cause it found.
• [ ] Nothing the test promises is weakened: no longer timeouts, no retries, no dropped assertion. Wait on the stage events the test means, never a sleep.
• [ ] If the cause is in the product (stages emitted out of order under load, say), fix the product and say so.

Files: test/view-source.test.ts and the module the cause is in. Why now: it sank both tries of #5354 (pulls #5377 and #5386), which never touched mirrors.