tighten transport failure semantics

main/trikke_usb_transport.c

@@ -26,6 +26,9 @@ static trikke_transport_status_t usb_begin_packet(
         return TRIKKE_TRANSPORT_PENDING;
     }
     if (written == 0) {
+        // Arguments and lifecycle were validated above, and this backend is the
+        // driver's sole owner. Under that invariant, zero means the all-or-none
+        // ring submission timed out without accepting this frame.
         return TRIKKE_TRANSPORT_RETRY;
     }
     return TRIKKE_TRANSPORT_FATAL;