fix: order command controls after dispatch

internal/server/session/session.go

@@ -26,6 +26,10 @@ type Frame struct {
 	// Written is closed by the sole socket writer after a successful write.
 	// It is used only for protocol barriers such as reconciliation-before-work.
 	Written chan<- struct{}
+	// OnWritten is a non-blocking session-local scheduler notification. It is
+	// invoked only after the sole socket writer has completed the frame, so a
+	// dependent control frame cannot overtake its command dispatch.
+	OnWritten func()
 }
 
 // WriterQueue is owned by one socket writer. Data saturation leaves the work