Accept live S300 frames with bad checksum

lib/core/bluetooth/bt_poller.dart

@@ -85,7 +85,11 @@ class BtPoller {
       final Uint8List frame =
           Uint8List.fromList(_rxBuf.sublist(startIdx, startIdx + 128));
 
-      if (validateFrame(frame)) {
+      final checksumOk = validateFrame(frame);
+      final canUseFrame = checksumOk || ecuType == EcuType.s300;
+
+      if (canUseFrame) {
+        if (!checksumOk) droppedFrames++;
         validFrames++;
         _frameController.add(frame);
         _rxBuf.removeRange(0, startIdx + 128);

lib/core/protocol/s300_parser.dart

@@ -4,12 +4,13 @@ import 'temp_table.dart';
 import 'neg8.dart';
 
 /// Parses a 128-byte S300 ECU response frame into a [SensorState].
-/// Throws [ArgumentError] if frame length is wrong or NEG8 checksum fails.
-SensorState parseS300(Uint8List frame) {
+/// Throws [ArgumentError] if frame length is wrong, or if [validateChecksum]
+/// is true and the NEG8 checksum fails.
+SensorState parseS300(Uint8List frame, {bool validateChecksum = true}) {
   if (frame.length != 128) {
     throw ArgumentError('S300 frame must be 128 bytes, got ${frame.length}');
   }
-  if (!validateFrame(frame)) {
+  if (validateChecksum && !validateFrame(frame)) {
     throw ArgumentError('S300 frame NEG8 checksum failed');
   }
 

lib/core/providers/sensor_provider.dart

@@ -14,7 +14,9 @@ final sensorStateProvider = StreamProvider<SensorState>((ref) {
   final btNotifier = ref.watch(btProvider.notifier);
 
   return btNotifier.frameStream.map((frame) {
-    if (ecuType == EcuType.s300) return parseS300(frame);
+    if (ecuType == EcuType.s300) {
+      return parseS300(frame, validateChecksum: false);
+    }
     return parseKPro(frame);
   });
 });
@@ -22,8 +24,6 @@ final sensorStateProvider = StreamProvider<SensorState>((ref) {
 /// Last successfully parsed sensor state (never null after first frame).
 /// Falls back to SensorState.zero() before any data arrives.
 final latestSensorProvider = Provider<SensorState>((ref) {
-  return ref
-      .watch(sensorStateProvider)
-      .whenData((s) => s)
-      .value ?? SensorState.zero();
+  return ref.watch(sensorStateProvider).whenData((s) => s).value ??
+      SensorState.zero();
 });

lib/ui/screens/debug/raw_frame_debug_screen.dart

@@ -51,7 +51,10 @@ class RawFrameDebugScreen extends ConsumerWidget {
               _RowData('Dropped', bt.droppedFrames.toString()),
               _RowData('Last frame bytes', frame?.length.toString() ?? 'none'),
               if (frame != null)
-                _RowData('Checksum', validateFrame(frame) ? 'valid' : 'bad'),
+                _RowData(
+                  'Checksum',
+                  validateFrame(frame) ? 'valid' : 'bad, accepted for S300',
+                ),
             ],
           ),
           const SizedBox(height: 12),

test/s300_parser_test.dart

@@ -152,5 +152,12 @@ void main() {
       frame[127] = 0x00; // corrupt checksum
       expect(() => parseS300(frame), throwsArgumentError);
     });
+
+    test('can parse bad checksum frame when validation is disabled', () {
+      final frame = _buildS300Frame(rpm: 1000);
+      frame[127] = 0x00; // corrupt checksum
+      final state = parseS300(frame, validateChecksum: false);
+      expect(state.rpm, equals(1000.0));
+    });
   });
 }