The fab-upload GC test stat-ed the .part immediately after observing the slot
free, but expireIdleUpload unlinks AFTER releasing the mutex. Passed alone,
failed in the full package once this release's render tests made web heavier.
Not a production defect - a new upload mints a fresh random .part. The test now
waits for the outcome it asserts on the same deadline; red-proofed by removing
the unlink from production.
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