fix: join the RNS loop thread before exit and keep the response slot per-request

core/shim/smolmail_rns.h

@@ -52,6 +52,14 @@ int smolmail_rns_request(const uint8_t *request, size_t request_len,
  * minutes (upstream spec sec 13.10). */
 void smolmail_rns_close(void);
 
+/* Stops the Reticulum stack: halts and joins the loop thread, tears the link
+ * down, deregisters and stops the UDP interface, and releases the
+ * Reticulum instance. The loop thread must be joined before the process (or
+ * embedding host) tears down statics, or it keeps calling into Reticulum
+ * while their destructors run. A no-op when the stack is not running.
+ * The stack can be started again afterwards. */
+void smolmail_rns_stop(void);
+
 #ifdef __cplusplus
 }
 #endif