The 5s probe fetched /metrics via asyncio.to_thread, which shares the
DEFAULT executor with second-long truncation tokenizations -- 96 of
those queue-jumped the probe and the state machine never ticked (gate
frozen at 1 while results flowed, ETA 6h). Probes now run on a
dedicated single-thread executor, and after 3 consecutive fetch
failures the gate stops asking for /metrics entirely (this endpoint
404s; pure demand mode from then on).
Verified under a choked default executor: gate ticks 1->2 on schedule
and metrics_dead engages after 3 failures.
Co-Authored-By: Claude <noreply@anthropic.com>
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