test(engine): hold the download attempt deadline so slow runners cannot trip it

[miguel-heygen]

What changes

Main went red on Windows in urlDownloader.test.ts ("locally refetches a caller checksum mismatch...") with Download timeout after 1s where hash_mismatch was expected.

Cause: every download attempt starts a real 1 second deadline timer, and the tests all pass 1_000. That test runs two attempts (fetch, hash, refetch, hash); on a slow Windows runner one attempt can outlast the timer. The earlier fix (#4155) faked only Date, so this timer was still real.

• The test file now holds that 1 second deadline timer instead of starting it. Other timers (cache-lock poll, test delays, the 20 ms stalled-body test) stay real. No timeout value is raised.
• "does not let one caller cancellation abort another caller" only passed because the real deadline fired after about 1 s (1078 ms on main). It now fires the held deadline by hand once the stalled first fetch has started.
• New test: a fetch that takes 1.2 s against a 1 s deadline still succeeds, and the stub rejects on abort, so an armed deadline would fail it.

No product code changes.

Verification

• urlDownloader.test.ts: 81 pass, three runs in a row on a remote Linux runner, exit 0 each. Lint and format clean.
• Deliberate mutation: turning the hold off makes the new test fail.
• Not exercised: the hosted Windows job (the flake is Windows-only, so the next Windows run is the real check).

Review

An independent read-only review found no blocking or should-fix items. One note: the hold matches a delay of exactly 1_000 ms, so a future test using 1_000 for something else would be held too.