feat(#144): gate 0xC1 (CMD_OFFBAND_GPS) to Offband-capable firmware

The fork-only 0xC1 GPS command was sent to any connected radio — both the
60s self-location poll (on gps=1) and the Settings GPS status query. Stock /
non-Offband MeshCore firmware can't answer it, so gate both behind a
capability check.

- firmwareSupportsOffbandGps(offbandCaps): presence of the Offband-fork
  offband_caps byte (device-info v14+), which stock MeshCore never emits.
- requestOffbandGps() hard-returns when unsupported — 0xC1 never goes out.
- _reconcileGpsPolling() centralizes the poll decision (support AND gps=1)
  across all three triggers + the device-info reply, so frame order is moot.
- Settings hides the GPS status section on unsupported firmware.

Interim presence-gate; a dedicated OFFBAND_CAP_GPS bit can follow once
firmware defines one. Unit-tested (3 new); Gemini review: ship.

Bumps build number to +43 (test build b43).

Closes #144

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

lib/connector/meshcore_connector.dart

@@ -460,6 +460,11 @@ class MeshCoreConnector extends ChangeNotifier {
   /// `offband_caps` capability bitfield from the device-info reply (v14+);
   /// null on older firmware that sends a shorter frame.
   int? get offbandCaps => _offbandCaps;
+
+  /// Whether the connected firmware speaks the `0xC1` GPS extension — i.e. it
+  /// advertised the Offband-fork `offband_caps` byte. Stock MeshCore omits it,
+  /// so 0xC1 is suppressed there rather than pinged blindly. (#144)
+  bool get supportsOffbandGps => firmwareSupportsOffbandGps(_offbandCaps);
   Map<String, String>? get currentCustomVars => _currentCustomVars;
   int? get batteryMillivolts => _batteryMillivolts;
   int? get storageUsedKb => _storageUsedKb;
@@ -2661,6 +2666,19 @@ class MeshCoreConnector extends ChangeNotifier {
     _gpsLocationPollTimer = null;
   }
 
+  /// Start or stop the 0xC1 GPS poll based on current state: only when the
+  /// radio reports `gps=1` AND the firmware advertises 0xC1 support. Called
+  /// from every input that can change either input (custom-var frames, the
+  /// enable toggle, and the device-info reply that delivers `offband_caps`) so
+  /// frame-arrival order doesn't matter. (#144)
+  void _reconcileGpsPolling() {
+    if (supportsOffbandGps && _currentCustomVars?['gps'] == '1') {
+      _startGpsLocationPolling();
+    } else {
+      _stopGpsLocationPolling();
+    }
+  }
+
   void setPollingInterval(int i) {
     _pollingInterval = i.clamp(1, 60);
     if (isConnected) {
@@ -3566,10 +3584,8 @@ class MeshCoreConnector extends ChangeNotifier {
       (_currentCustomVars ??= <String, String>{})[key] = val;
       notifyListeners();
     }
-    if (value == 'gps:1') {
-      _startGpsLocationPolling();
-    } else if (value == 'gps:0') {
-      _stopGpsLocationPolling();
+    if (value == 'gps:1' || value == 'gps:0') {
+      _reconcileGpsPolling();
     }
   }
 
@@ -3824,6 +3840,8 @@ class MeshCoreConnector extends ChangeNotifier {
     Duration timeout = const Duration(seconds: 5),
   }) async {
     if (!isConnected) return null;
+    // Never emit the fork-only 0xC1 to firmware that can't answer it. (#144)
+    if (!supportsOffbandGps) return null;
     final existing = _offbandGpsCompleter;
     if (existing != null && !existing.isCompleted) {
       // A query is already in flight — share it rather than orphaning it.
@@ -4213,6 +4231,9 @@ class MeshCoreConnector extends ChangeNotifier {
     // Offband config capability v14+ (byte 82). Extracted + bounds-checked in a
     // testable helper; offset verified against firmware (see parseOffbandCaps).
     _offbandCaps = parseOffbandCaps(frame);
+    // Caps just landed; (re)evaluate GPS polling in case a `gps=1` custom-var
+    // frame arrived before this device-info reply set support. (#144)
+    _reconcileGpsPolling();
 
     // Firmware reports MAX_CONTACTS / 2 for v3+ device info.
     final reportedContacts = frame[2];
@@ -6301,13 +6322,10 @@ class MeshCoreConnector extends ChangeNotifier {
     final buf = BufferReader(frame.sublist(1));
     try {
       _currentCustomVars = _parseKeyValueString(buf.readCString());
-      // Reflect current GPS state in the polling timer (handles initial
-      // device state on connect as well as external CLI/USB toggles).
-      if (_currentCustomVars?['gps'] == '1') {
-        _startGpsLocationPolling();
-      } else {
-        _stopGpsLocationPolling();
-      }
+      // Reflect current GPS state in the polling timer (handles initial device
+      // state on connect as well as external CLI/USB toggles); gated on 0xC1
+      // support so stock firmware never gets polled. (#144)
+      _reconcileGpsPolling();
     } catch (e) {
       appLogger.warn('Malformed custom vars frame: $e', tag: 'Connector');
     }

lib/connector/meshcore_protocol.dart

@@ -263,6 +263,17 @@ const int respCodeOffbandGps = 0xC1;
 /// Request frame for [cmdOffbandGps] — a bare 1-byte command, no payload. (#135)
 Uint8List buildOffbandGpsRequestFrame() => Uint8List.fromList([cmdOffbandGps]);
 
+/// True iff the connected firmware understands the `0xC1` GPS extension. Gated
+/// on the `offband_caps` byte being present: that byte is an Offband-fork
+/// addition (device-info v14+) which stock/upstream MeshCore never emits, so a
+/// null caps value means non-Offband firmware that couldn't answer `0xC1` — and
+/// must not be pinged with it. (#144)
+///
+/// Interim presence-gate: a dedicated `OFFBAND_CAP_GPS` bit (firmware
+/// follow-up) would let an Offband build without GPS opt out; until firmware
+/// defines one, any Offband v14+ radio is assumed to speak `0xC1`.
+bool firmwareSupportsOffbandGps(int? offbandCaps) => offbandCaps != null;
+
 const int statsTypeCore = 0;
 const int statsTypeRadio = 1;
 const int statsTypePackets = 2;

lib/screens/settings_screen.dart

@@ -886,6 +886,7 @@ class _SettingsScreenState extends State<SettingsScreen> {
                     }
                   },
                 ),
+                if (connector.supportsOffbandGps)
                   _buildGpsQuerySection(
                     context,
                     gpsStatus,

pubspec.yaml

@@ -16,7 +16,7 @@ publish_to: 'none' # Remove this line if you wish to publish to pub.dev
 # https://developer.apple.com/library/archive/documentation/General/Reference/InfoPlistKeyReference/Articles/CoreFoundationKeys.html
 # In Windows, build-name is used as the major, minor, and patch parts
 # of the product and file versions while build-number is used as the build suffix.
-version: 1.1.2-beta.4+36
+version: 1.1.2-beta.4+43
 
 environment:
   sdk: ^3.9.2

test/offband_gps_capability_test.dart

@@ -0,0 +1,21 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:meshcore_open/connector/meshcore_protocol.dart';
+
+void main() {
+  group('firmwareSupportsOffbandGps', () {
+    test('null caps (stock/pre-v14 MeshCore omits the byte) → unsupported', () {
+      expect(firmwareSupportsOffbandGps(null), isFalse);
+    });
+
+    test('caps byte present with no bits set → supported', () {
+      // An Offband v14+ radio that simply isn't a wifi observer still emits the
+      // offband_caps byte (value 0), so it speaks the 0xC1 fork extension.
+      expect(firmwareSupportsOffbandGps(0x00), isTrue);
+    });
+
+    test('caps byte present with bits set → supported', () {
+      expect(firmwareSupportsOffbandGps(0x01), isTrue);
+      expect(firmwareSupportsOffbandGps(0xFF), isTrue);
+    });
+  });
+}