Windows: the outcome deadline test can elapse before its reader starts #511

Problem

windows-test failed twice in a row on #509 (run 40940288086, attempts 1 and 2) in a test that #509 does not touch:

FAILED tests/ui/test_deployment_outcome_controller.py::test_absolute_deadline_reaps_reader_that_suppresses_cancellation - TimeoutError
tests/ui/test_deployment_outcome_controller.py:914: await asyncio.wait_for(started.wait(), timeout=HANG_GUARD_SECONDS)

Cause

The test builds the controller with deadline_seconds=0.01 and waits for the reader to start. DeploymentOutcomeController._observe computes remaining = deadline_seconds - (clock() - started) before the first read, and returns with the deadline marked elapsed when remaining <= 0. On a loaded runner, more than 10 ms passes between registering the observer and the worker running, so the reader is never called. started is then never set, and the test times out after 30 s.

So the test asserts a timing it does not control. Production is unaffected: the default deadline is 300 s.

Done when

The test reaches the reader before the deadline can elapse, independent of runner speed, by controlling the clock the controller reads rather than relying on 10 ms of wall time. It still proves that the absolute deadline reaps a reader that suppresses cancellation.