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/__init__.py

@@ -9,7 +9,7 @@ from typing import Any
 import voluptuous as vol
 
 from homeassistant.config_entries import ConfigEntry
-from homeassistant.const import Platform
+from homeassistant.const import EVENT_HOMEASSISTANT_STOP, Platform
 from homeassistant.core import (
     HomeAssistant,
     ServiceCall,
@@ -121,6 +121,16 @@ async def async_setup_entry(hass: HomeAssistant, entry: ConfigEntry) -> bool:
         )
     )
 
+    # Persist RNS/LXMF state off-loop when Home Assistant stops (we unregister
+    # RNS's own atexit handlers, which would otherwise do this with blocking I/O
+    # on the event loop and detach HA's stdout).
+    async def _on_ha_stop(_event) -> None:
+        await manager.async_persist()
+
+    runtime.unsubs.append(
+        hass.bus.async_listen_once(EVENT_HOMEASSISTANT_STOP, _on_ha_stop)
+    )
+
     entry.async_on_unload(entry.add_update_listener(_async_options_updated))
     _async_register_services(hass)