feat(dfu): add packet codec and crc32 utilities

test/service/dfu_protocol_test.dart

@@ -0,0 +1,86 @@
+import 'package:abawo_bt_app/model/shifter_types.dart';
+import 'package:abawo_bt_app/service/dfu_protocol.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  group('DfuProtocol CRC32', () {
+    test('matches known vector', () {
+      final crc = DfuProtocol.crc32('123456789'.codeUnits);
+      expect(crc, 0xCBF43926);
+    });
+  });
+
+  group('DfuProtocol control payload encoding', () {
+    test('encodes START payload with exact 11-byte LE layout', () {
+      final payload = DfuProtocol.encodeStartPayload(
+        const DfuStartPayload(
+          totalLength: 0x1234,
+          imageCrc32: 0x89ABCDEF,
+          sessionId: 0x22,
+          flags: universalShifterDfuFlagEncrypted,
+        ),
+      );
+
+      expect(payload.length, 11);
+      expect(
+        payload,
+        [
+          universalShifterDfuOpcodeStart,
+          0x34,
+          0x12,
+          0x00,
+          0x00,
+          0xEF,
+          0xCD,
+          0xAB,
+          0x89,
+          0x22,
+          universalShifterDfuFlagEncrypted,
+        ],
+      );
+    });
+
+    test('encodes FINISH and ABORT payloads as one byte', () {
+      expect(
+          DfuProtocol.encodeFinishPayload(), [universalShifterDfuOpcodeFinish]);
+      expect(
+          DfuProtocol.encodeAbortPayload(), [universalShifterDfuOpcodeAbort]);
+    });
+  });
+
+  group('DfuProtocol data frame building', () {
+    test('builds 64-byte frames and handles final partial payload', () {
+      final image = List<int>.generate(80, (index) => index);
+      final frames = DfuProtocol.buildDataFrames(image);
+
+      expect(frames.length, 2);
+
+      expect(frames[0].sequence, 0);
+      expect(frames[0].offset, 0);
+      expect(frames[0].payloadLength, universalShifterDfuFramePayloadSizeBytes);
+      expect(frames[0].bytes.length, universalShifterDfuFrameSizeBytes);
+      expect(frames[0].bytes.sublist(1, 64), image.sublist(0, 63));
+
+      expect(frames[1].sequence, 1);
+      expect(frames[1].offset, 63);
+      expect(frames[1].payloadLength, 17);
+      expect(frames[1].bytes.length, universalShifterDfuFrameSizeBytes);
+      expect(frames[1].bytes.sublist(1, 18), image.sublist(63, 80));
+    });
+  });
+
+  group('DfuProtocol sequence and ACK helpers', () {
+    test('wraps sequence values and computes ack+1 rewind', () {
+      expect(DfuProtocol.nextSequence(0x00), 0x01);
+      expect(DfuProtocol.nextSequence(0xFF), 0x00);
+
+      expect(DfuProtocol.rewindSequenceFromAck(0x05), 0x06);
+      expect(DfuProtocol.rewindSequenceFromAck(0xFF), 0x00);
+    });
+
+    test('computes wrapping sequence distance', () {
+      expect(DfuProtocol.sequenceDistance(250, 2), 8);
+      expect(DfuProtocol.sequenceDistance(1, 1), 0);
+    });
+  });
+}