Harden against crafted-packet crashes + adversarial fuzzing (#10862)

Audit and fuzzing of the RF-packet decode -> dispatch -> display/phone paths for
the "crash a node or phone with a crafted packet" surface, beyond the XEdDSA
authenticity work.

Crash fixes (reproduced under AddressSanitizer / UBSan):
- GeoCoord::latLongToUTM/latLongToMGRS read fixed letter tables out of bounds on
  extreme latitude_i/longitude_i from a received Position, and narrowed
  out-of-range easting/northing doubles to unsigned (float-cast-overflow UB).
  Clamp the UTM zone, the easting/northing narrowing, and the band/col/row
  indices. Regression: test_geocoord_extreme_coords_no_oob.
- EnvironmentTelemetry/AirQualityTelemetry render attacker floats via
  String(float), which on nRF52/RP2040/STM32/portduino formats into a fixed
  char[33] (dtostrf) and overflows near FLT_MAX. Clamp the rendered metrics via
  UnitConversions::displaySafeFloat (finite + magnitude <= 1e9), unit-tested in
  test_type_conversions.

Defense-in-depth + robustness:
- TraceRouteModule::printRoute: fix an snr_back[-1] OOB read (wrong count in the
  guard) and stop formatting the INT8_MIN "unknown SNR" sentinel as a dB value.
- WaypointModule/NodeDB: sanitize untrusted strings before the OLED renderer and
  the phone-facing ClientNotification (belt-and-suspenders vs PB_VALIDATE_UTF8).
- MeshService::sendToPhone: withhold NODEINFO/WAYPOINT packets whose nested string
  won't cleanly decode, protecting strict phone protobuf decoders without
  affecting mesh relay.

Tests: new test_fuzz_decode (protobuf decode + UTF-8 sanitizer fuzz) and
test_fuzz_packets (perhapsDecode / module-handler / traceroute / phone-gate fuzz),
all under AddressSanitizer; native-suite-count 25 -> 27. Full suite 515/515 green.

src/modules/TraceRouteModule.cpp

@@ -452,7 +452,7 @@ void TraceRouteModule::printRoute(meshtastic_RouteDiscovery *r, uint32_t origin,
     // If there's a route back (or we are the destination as then the route is complete), print it
     if (r->route_back_count > 0 || origin == nodeDB->getNodeNum()) {
         route += "\n";
-        if (r->snr_towards_count > 0 && origin == nodeDB->getNodeNum())
+        if (origin == nodeDB->getNodeNum() && r->snr_back_count > 0 && r->snr_back[r->snr_back_count - 1] != INT8_MIN)
             route += vformat("(%.2fdB) 0x%x <-- ", (float)r->snr_back[r->snr_back_count - 1] / 4, origin);
         else
             route += "...";