support milestone and thrash rotation signals

internal/orchestrator/orchestrator.go

@@ -161,8 +161,14 @@ func (c *Coordinator) rotate(ctx context.Context, hard float64) {
 		if err != nil {
 			continue
 		}
+		reason := "threshold"
+		if signal, ok := a.(herdr.RotationSignal); ok {
+			if r, signalErr := signal.RotationSignal(ctx, session); signalErr == nil && r != "" {
+				reason = r
+			}
+		}
 		occupancy, err := a.Occupancy(session)
-		if err != nil || occupancy < hard {
+		if err != nil || (occupancy < hard && reason == "threshold") {
 			continue
 		}
 		if boundary, ok := a.(herdr.TurnBoundary); ok {
@@ -178,7 +184,7 @@ func (c *Coordinator) rotate(ctx context.Context, hard float64) {
 		if ref == "" {
 			continue
 		}
-		b, _ := json.Marshal(map[string]string{"handoff_ref": ref, "reason": "threshold"})
+		b, _ := json.Marshal(map[string]string{"handoff_ref": ref, "reason": reason})
 		e := domain.Event{ID: domain.NewID(), Type: "TaskReleased", TaskID: taskID, Version: task.Version + 1, Payload: b}
 		if c.Store.Append(e) == nil {
 			c.mu.Lock()