fix: lat/lon double-write in buildUpdateContactPathFrame

The function emitted two consecutive 8-byte position blocks instead of
one, producing a frame 8 bytes longer than the documented layout. When
a caller passed lastModified, the firmware parsed the duplicated second
lat as the timestamp, giving wildly wrong "last seen" values on
imported contacts.

Delete the unconditional first block; keep the conditional block that
correctly skips the optional tail when neither location nor
lastModified is set, zero-fills position slots when only lastModified
is present, and appends the optional timestamp.

Adds test/connector/build_update_contact_path_frame_test.dart with
five cases covering all four optional-tail combinations plus the
fixed-point lat/lon encoding.

Fixes #427

lib/connector/meshcore_protocol.dart

@@ -720,27 +720,16 @@ Uint8List buildUpdateContactPathFrame(
   final timestamp = DateTime.now().millisecondsSinceEpoch ~/ 1000;
   writer.writeUInt32LE(timestamp);
 
-  if ((lat == null || lon == null) && lastModified != null) {
-    // If lat/lon not provided, write zeros
-    writer.writeInt32LE(0);
-    writer.writeInt32LE(0);
-  } else {
-    // Latitude and Longitude are expected in degrees, convert to int by multiplying by 1e6
-    // Latitude
-    final latitude = lat ?? 0.0;
-    writer.writeInt32LE((latitude * 1e6).round());
-
-    // Longitude
-    final longitude = lon ?? 0.0;
-    writer.writeInt32LE((longitude * 1e6).round());
-  }
-
+  // Optional [Lat x4, Lon x4][timestamp x4] tail per the doc comment above.
+  // Emit 8 bytes of position (zero-filled when only lastModified is provided)
+  // followed by an optional 4-byte timestamp. Earlier code emitted the
+  // position block twice, which corrupted the tail and caused the firmware
+  // to parse the second lat as the timestamp. See #427.
   final hasLocation = lat != null && lon != null;
   if (hasLocation || lastModified != null) {
     writer.writeInt32LE(hasLocation ? (lat * 1e6).round() : 0);
     writer.writeInt32LE(hasLocation ? (lon * 1e6).round() : 0);
     if (lastModified != null) {
-      // Last modified
       final lastModifiedTimestamp = lastModified.millisecondsSinceEpoch ~/ 1000;
       writer.writeUInt32LE(lastModifiedTimestamp);
     }

test/connector/build_update_contact_path_frame_test.dart

@@ -0,0 +1,105 @@
+import 'dart:typed_data';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:meshcore_open/connector/meshcore_protocol.dart';
+
+void main() {
+  // Frame layout per the doc comment on buildUpdateContactPathFrame:
+  //   [cmd][pub_key x32][type][flags][path_len][path x64][name x32]
+  //   [timestamp x4][Lat? x4, Lon? x4][timestamp? x4]
+  //
+  // Base (mandatory) bytes:
+  //   1 cmd + 32 pubKey + 1 type + 1 flags + 1 pathLen + 64 path
+  //   + 32 name + 4 timestamp = 136 bytes
+  const int baseFrameLength = 136;
+
+  final pubKey = Uint8List.fromList(List<int>.generate(32, (i) => i));
+  final path = Uint8List.fromList([0xAA, 0xBB]);
+
+  group('buildUpdateContactPathFrame', () {
+    test('omits lat/lon and timestamp tail when neither is provided', () {
+      final frame = buildUpdateContactPathFrame(
+        pubKey,
+        path,
+        path.length,
+        name: 'Alice',
+      );
+
+      // Should be exactly the base frame, no optional tail.
+      expect(frame.length, baseFrameLength);
+    });
+
+    test('appends only an 8-byte lat/lon tail when location is provided', () {
+      final frame = buildUpdateContactPathFrame(
+        pubKey,
+        path,
+        path.length,
+        lat: 49.123456,
+        lon: -123.123456,
+      );
+
+      expect(frame.length, baseFrameLength + 8);
+    });
+
+    test(
+      'appends 8 bytes lat/lon + 4 bytes timestamp when both are provided',
+      () {
+        final frame = buildUpdateContactPathFrame(
+          pubKey,
+          path,
+          path.length,
+          lat: 49.0,
+          lon: -123.0,
+          lastModified: DateTime.utc(2026, 1, 2, 3, 4, 5),
+        );
+
+        expect(frame.length, baseFrameLength + 8 + 4);
+      },
+    );
+
+    test('zero-fills the lat/lon slots and appends timestamp when only '
+        'lastModified is provided', () {
+      final frame = buildUpdateContactPathFrame(
+        pubKey,
+        path,
+        path.length,
+        lastModified: DateTime.utc(2026, 1, 2, 3, 4, 5),
+      );
+
+      // 8 zero bytes for lat/lon + 4 bytes timestamp
+      expect(frame.length, baseFrameLength + 8 + 4);
+
+      // Verify the lat/lon slot is actually zero — guards against a
+      // regression where the function writes garbage into those bytes.
+      final tailStart = baseFrameLength;
+      for (var i = tailStart; i < tailStart + 8; i++) {
+        expect(frame[i], 0, reason: 'byte $i in lat/lon slot must be 0');
+      }
+    });
+
+    test('encodes positive lat/lon as little-endian fixed-point (×1e6)', () {
+      final frame = buildUpdateContactPathFrame(
+        pubKey,
+        path,
+        path.length,
+        lat: 49.123456,
+        lon: -123.123456,
+      );
+
+      // Latitude is the first 4 bytes of the optional tail.
+      final latBytes = ByteData.sublistView(
+        frame,
+        baseFrameLength,
+        baseFrameLength + 4,
+      );
+      final lonBytes = ByteData.sublistView(
+        frame,
+        baseFrameLength + 4,
+        baseFrameLength + 8,
+      );
+
+      expect(latBytes.getInt32(0, Endian.little), (49.123456 * 1e6).round());
+      expect(lonBytes.getInt32(0, Endian.little), (-123.123456 * 1e6).round());
+    });
+  });
+}