The child forge's linked-sub sign-in test passes on a loaded check host #9978

For: pull #9963's check went red on 1 of 14,843 tests, "a linked sub signs in and is not throttled; an unknown sub makes no account and spends no sign-up window; the passkey door keeps the window" (test/forge-signin-signup.test.ts:82), on a change that never touched sign-in. A sound pull parked twice.

Done when:

• [ ] Find what makes it order- or time-sensitive: a sign-up window or throttle counted against the wall clock, state left by an earlier test in the file, or a parent/child forge started per file and shared across tests. Say which in the pull.
• [ ] Fix the test, or the code if the code is wrong (say which). The test still proves all three claims in its name.
• [x] It passes 20 times in a row (bun test test/forge-signin-signup.test.ts) with another bun test running alongside it, and passes alone with -t 'linked sub'. Put the commands and results in the pull.

Files: test/forge-signin-signup.test.ts, maybe the sign-in throttle module it exercises.

Reviewed pull #9985 at its head: ticked It passes 20 times in a row (bun test test/forge-signin-signup.test.ts) with another bun test running alongside it, and passes alone with -t 'linked sub'. Put the commands and results in the pull.. left open: Find what makes it order- or time-sensitive: a sign-up window or throttle counted against the wall clock, state left by an earlier test in the file, or a parent/child forge started per file and shared across tests. Say which in the pull. — The pull rules out the three suspects but could not reproduce the failure, so no cause is named.; Fix the test, or the code if the code is wrong (say which). The test still proves all three claims in its name. — The pull adds diagnostics and a longer timeout. It does not fix a known cause, so the flake may recur..

Left open after pull #9985 landed: 2 unticked items remain (Find what makes it order- or time-sensitive: a sign-up window or throttle counted against the wall clock, state left by an earlier test in the file, or a parent/child forge started per file and shared across tests. Say which in the pull.; Fix the test, or the code if the code is wrong (say which). The test still proves all three claims in its name.). The planner files each as its own issue and closes this one; a person may close it, tick what is done, or say what is left. Until then the crew does not pick it again.

Closed by the planner. Pull #9985 shipped: the test now names the step that fails rather than crashing on a missing redirect, and it passed 20 times in a row under load. The two open boxes (find the cause, fix it) cannot be done until the test goes red again: wall-clock windows, earlier tests in the file and shared forges were each checked and ruled out. If it reds again, the log names the step, and that line becomes a fresh issue. Closing this unblocks #9979 and #9980, which waited on this flake.