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/mesh/MeshService.h

@@ -100,6 +100,11 @@ class MeshService
                p->decoded.portnum == meshtastic_PortNum_DETECTION_SENSOR_APP ||
                p->decoded.portnum == meshtastic_PortNum_ALERT_APP;
     }
+
+    /// Returns false when a decoded NodeInfo/Waypoint payload fails nested protobuf decode (invalid
+    /// UTF-8 under PB_VALIDATE_UTF8, etc.); other portnums pass through. Callers gate on the variant.
+    static bool phonePayloadIsDecodable(const meshtastic_Data &decoded);
+
     /// Called when some new packets have arrived from one of the radios
     Observable<uint32_t> fromNumChanged;