engine: require sustained late writes before degrading runtime

internal/app/engine.go

@@ -113,13 +113,14 @@ type RuntimeTransition struct {
 }
 
 const (
-	lateBufferIndicatorWindow        = 5 * time.Second
-	writeLateTolerance               = 1 * time.Millisecond
+	lateBufferIndicatorWindow        = 2 * time.Second
+	writeLateTolerance               = 10 * time.Millisecond
 	queueCriticalStreakThreshold     = 3
 	queueMutedStreakThreshold        = queueCriticalStreakThreshold * 2
 	queueMutedRecoveryThreshold      = queueCriticalStreakThreshold
 	queueFaultedStreakThreshold      = queueCriticalStreakThreshold
 	faultRepeatWindow                = 1 * time.Second
+	lateBufferStreakThreshold        = 3   // consecutive late writes required before alerting
 	faultHistoryCapacity             = 8
 	runtimeTransitionHistoryCapacity = 8
 )
@@ -150,6 +151,7 @@ type Engine struct {
 	underruns           atomic.Uint64
 	lateBuffers         atomic.Uint64
 	lateBufferAlertAt   atomic.Uint64
+	lateBufferStreak    atomic.Uint64 // consecutive late writes; reset on clean write
 	criticalStreak      atomic.Uint64
 	mutedRecoveryStreak atomic.Uint64
 	mutedFaultStreak    atomic.Uint64
@@ -604,12 +606,23 @@ func (e *Engine) writerLoop(ctx context.Context) {
 
 		lateOver := writeDur - e.chunkDuration
 		if lateOver > writeLateTolerance {
+			streak := e.lateBufferStreak.Add(1)
 			late := e.lateBuffers.Add(1)
-			e.lateBufferAlertAt.Store(uint64(time.Now().UnixNano()))
+			// Only arm the alert window once the streak threshold is reached.
+			// Isolated OS-scheduling or USB jitter spikes (single late writes)
+			// are normal on a loaded system and must not trigger degraded state.
+			// This mirrors the queue-health streak logic.
+			if streak >= lateBufferStreakThreshold {
+				e.lateBufferAlertAt.Store(uint64(time.Now().UnixNano()))
+			}
 			if late <= 5 || late%20 == 0 {
-				log.Printf("TX LATE: write=%s budget=%s over=%s tolerance=%s queueResidence=%s pipeline=%s",
-					writeDur, e.chunkDuration, lateOver, writeLateTolerance, queueResidence, pipelineLatency)
+				log.Printf("TX LATE [streak=%d]: write=%s budget=%s over=%s tolerance=%s queueResidence=%s pipeline=%s",
+					streak, writeDur, e.chunkDuration, lateOver, writeLateTolerance, queueResidence, pipelineLatency)
 			}
+		} else {
+			// Clean write — reset the consecutive streak so isolated spikes
+			// never accumulate toward the threshold.
+			e.lateBufferStreak.Store(0)
 		}
 
 		if err != nil {