test/snapshot-mirror.test.ts's off-box mirror cases pass every time on a loaded check machine #4624

The off-box mirror cases in test/snapshot-mirror.test.ts now wait on events instead of a fixed sleep. I couldn't reproduce the original red on a loaded machine, so the cause I fixed is from reading the code. Only the test changed, not src/.

• Upload: "a snapshot lands in both buckets" and "an unreachable mirror is one line on the board" now call svc.mirrorFresh() after svc.now(). now() starts the mirror upload in the background, and mirrorFresh() runs a pass only once any pass already in flight has ended, so the test sees the mirror as it really is.
• Notifier: the "unreachable mirror" case had clockMs: 20 and Bun.sleep(40) to wait for the notifier's interval. On a loaded machine that interval can fire late or coalesce, which leaves sent empty. The interval is now set to an hour, so it never fires. Each wake is a drill.finished bus event that the test sends and then awaits with n.settled(). The "one push, however many wakes" assertion is unchanged. No time budget is left, so the load-proof helper from #4431 isn't needed.

How it was run: the three type-check and test commands below passed, but I did not run scripts/flake-hunt.ts.

• bun x tsc --noEmit is clean.
• bun test test/snapshot-mirror.test.ts passed in 20 copies at once, all exit 0.
• Before the change, 12 simultaneous copies also all passed.

--randomize fails this file because its tests share state and depend on file order. I didn't change that, and the forge's check doesn't randomize this file's test order, as far as I could tell.

Closes #4580

The off-box mirror cases in test/snapshot-mirror.test.ts now wait on events instead of a fixed sleep. I couldn't reproduce the original red on a loaded machine, so the cause I fixed is from reading the code. Only the test changed, not src/.

• Upload: "a snapshot lands in both buckets" and "an unreachable mirror is one line on the board" now call svc.mirrorFresh() after svc.now(). now() starts the mirror upload in the background, and mirrorFresh() runs a pass only once any pass already in flight has ended, so the test sees the mirror as it really is.
• Notifier: the "unreachable mirror" case had clockMs: 20 and Bun.sleep(40) to wait for the notifier's interval. On a loaded machine that interval can fire late or coalesce, which leaves sent empty. The interval is now set to an hour, so it never fires. Each wake is a drill.finished bus event that the test sends and then awaits with n.settled(). The "one push, however many wakes" assertion is unchanged. No time budget is left, so the load-proof helper from #4431 isn't needed.

How it was run: the three type-check and test commands below passed, but I did not run scripts/flake-hunt.ts.

• bun x tsc --noEmit is clean.
• bun test test/snapshot-mirror.test.ts passed in 20 copies at once, all exit 0.
• Before the change, 12 simultaneous copies also all passed.

--randomize fails this file because its tests share state and depend on file order. I didn't change that, and the forge's check doesn't randomize this file's test order, as far as I could tell.