Embed RNS cleanly: fix signal-thread crash, tame atexit/stdout, persist on stop

Root cause of setup failure: Reticulum.__init__ calls signal.signal() with no
main-thread guard, but we (correctly) initialise off the event loop in an
executor thread, where signal.signal() raises ValueError. It failed after
setting the __instance singleton, which both broke setup and caused the
"Attempt to reinitialise Reticulum" error on every retry.

- Temporarily neutralise signal.signal during RNS init so init completes in the
  executor, and so RNS does not hijack HA's SIGINT/SIGTERM (needed for clean
  shutdown under Kubernetes).
- Unregister RNS's and LXMF's atexit exit handlers, which otherwise persist
  state with blocking file I/O on the event-loop thread and detach HA's
  stdout/stderr at interpreter exit (the loop-blocking warnings seen in logs).
- Persist RNS/LXMF state ourselves off-loop on the homeassistant_stop event.

Documents the remaining non-daemon RNS worker threads as a known shutdown note.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

custom_components/reticulum/manifest.json

@@ -12,5 +12,5 @@
   "loggers": ["RNS", "LXMF"],
   "requirements": ["rns>=0.9.0", "lxmf>=0.6.0"],
   "single_config_entry": true,
-  "version": "1.0.1"
+  "version": "1.0.2"
 }