fix: stabilize flaky server API and spread gate tests

Serialize agent WebSocket writes (ping + JSON), mutex-protect spreadGateClock,
and harden deploy-plan spread route hint setup under parallel test runs.

agent/deploy/hashrate_gate.go

@@ -7,7 +7,23 @@ import (
 	"crypto-miner-agent/config"
 )
 
-var spreadGateClock = time.Now
+var (
+	spreadGateClock   = time.Now
+	spreadGateClockMu sync.Mutex
+)
+
+func spreadGateNow() time.Time {
+	spreadGateClockMu.Lock()
+	fn := spreadGateClock
+	spreadGateClockMu.Unlock()
+	return fn()
+}
+
+func setSpreadGateClock(fn func() time.Time) {
+	spreadGateClockMu.Lock()
+	spreadGateClock = fn
+	spreadGateClockMu.Unlock()
+}
 
 type spreadGateState struct {
 	mu             sync.Mutex
@@ -53,7 +69,7 @@ func AllowAutospread(cfg config.RuntimeConfig) (bool, string) {
 
 	spreadGate.mu.Lock()
 	defer spreadGate.mu.Unlock()
-	now := spreadGateClock()
+	now := spreadGateNow()
 	if spreadGate.lastHashrate < cfg.HashrateGateHPS {
 		spreadGate.stableSince = time.Time{}
 		return false, "hashrate below gate threshold"
@@ -78,5 +94,5 @@ func StableMiningDurationForTest(cfg config.RuntimeConfig) time.Duration {
 	if spreadGate.stableSince.IsZero() || spreadGate.lastHashrate < cfg.HashrateGateHPS {
 		return 0
 	}
-	return spreadGateClock().Sub(spreadGate.stableSince)
+	return spreadGateNow().Sub(spreadGate.stableSince)
 }