Make ckpool client resilient to warm-up EOFs

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.

api/internal/state/aggregator.go

@@ -59,6 +59,12 @@ type Aggregator struct {
 	mu     sync.RWMutex
 	snap   Snapshot
 	blocks []BlockRecord
+
+	// ckFailStreak counts consecutive refreshes where CKPool returned an
+	// error. The first failure after a streak of successes is logged at
+	// DEBUG (likely a transient warm-up or lock hiccup); repeated failures
+	// escalate to WARN.
+	ckFailStreak int
 }
 
 func New(ck *ckpool.Client, rpc *bitcoind.RPC, interval time.Duration, log *slog.Logger) *Aggregator {
@@ -108,8 +114,17 @@ func (a *Aggregator) refresh(ctx context.Context) {
 		next.HashrateHs1h = ckpool.DSPSToHashrate(ps.DSPS60)
 		next.HashrateHs24h = ckpool.DSPSToHashrate(ps.DSPS1440)
 		next.CKPoolOK = true
+		a.ckFailStreak = 0
 	} else {
-		a.Log.Warn("ckpool poolstats failed", "err", err)
+		a.ckFailStreak++
+		// First failure or two: likely transient (stratifier warm-up or
+		// a lock hiccup during a reconnect). Only escalate to WARN after
+		// three consecutive failures.
+		if a.ckFailStreak >= 3 {
+			a.Log.Warn("ckpool poolstats failed", "err", err, "streak", a.ckFailStreak)
+		} else {
+			a.Log.Debug("ckpool poolstats transient error", "err", err, "streak", a.ckFailStreak)
+		}
 		next.LastError = err.Error()
 	}
 	if u, err := a.CK.Uptime(ctx); err == nil {