A dead mirror emitted zero bytes and the fixed 3600s attempt timeout
let it stall the whole chain for an hour (third occurrence tonight:
15m49s frozen at 0 bytes). select() with a 300s idle timeout now kills
the attempt and moves to the next mirror; a trickling source (1 line/s
Downloading updates) resets the timer and stays alive.
Co-Authored-By: Claude <noreply@anthropic.com>