CI only: Windows deadline test waits for readiness

[basal-alfonso]

CI-only draft for a worker's Windows proof. Not for merging; the change reaches main through the gated push.

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Summary by cubic

Makes the Windows subprocess deadline tests reliable by starting the output deadline only after confirming the descendant process is actually running.

• Adds output_until_ready, which polls a readiness predicate on Windows before starting the output deadline timer.
• Updates the Windows pipe tests to wait for the descendant PID to be live instead of assuming it starts immediately.
• Adds a guarded mutation proving the deadline test fails when readiness gating is removed.

^{Written for commit 33478e8. Summary will update on new commits.}

[basal-alfonso]

Closing: CI-only branch; its change reaches main through the gated push.