The stratifier can accept a connection and close it without
writing a response during early startup — the refresh loop
then logs 'ckpool poolstats failed: read len: EOF' on every
tick until the stratifier is ready, which is noisy and looks
like a real fault.
- client.Send retries once after 200ms if the server closed
the connection before any bytes were read (io.EOF anywhere
in the wrapped chain).
- Aggregator tracks a ckFailStreak counter: first two
consecutive failures log at DEBUG, third and beyond escalate
to WARN. Successful refreshes reset the streak.