Fix "Attempt to reinitialise Reticulum" on setup/retry

RNS.Reticulum.__init__ sets its internal singleton before it can fail partway
through init, which left our module-level _RNS_INSTANCE cache out of sync (None)
while RNS believed it was already running. Every setup retry then hit RNS's
reinit guard and raised "Attempt to reinitialise Reticulum, when it was already
running".

Use RNS.Reticulum.get_instance() as the source of truth and adopt an existing
instance instead of re-creating it (with an OSError fallback that also adopts).
Also cache and reuse the LXMF router, delivery destination, identity and
announce handler across reloads so a reload never spawns a duplicate router or
stacks announce handlers.

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": "1.0.0"
+  "version": "1.0.1"
 }