add a serial test lane for concurrency-vulnerable tests

Generalizes the pty split into an explicit "parallel=false" opt-out: a
`serial` tag (declared in dart_test.yaml). The parallel flutter run now
excludes `pty || serial`; a separate `flutter test --tags serial
--concurrency=1` pass runs the vulnerable ones. For the coverage gate the
two passes are real-merged by ci/merge_lcov.py (union DA, max hits, recompute
LF/LH) — a plain concat would double-count and corrupt the total.

Tag transcript_publisher's bus-republish test serial (it flaked in the
parallel pool). Gate verified green end-to-end at 95.08%.

T-193.

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

test/builtin/claude/transcript_publisher_test.dart

@@ -44,7 +44,9 @@ void main() {
     setUp(() async => base = await Directory.systemTemp.createTemp('transcript_publisher_test_'));
     tearDown(() async => base.delete(recursive: true));
 
-    test('republishes reader items onto the bus (lead channel + item key)', () async {
+    // Serialized: this MessageBus republish assertion is timing-sensitive and
+    // flaked in the parallel flutter pool; runs in the --concurrency=1 pass (T-193).
+    test('republishes reader items onto the bus (lead channel + item key)', tags: ['serial'], () async {
       final dir = Directory('${base.path}/${workspace.replaceAll('/', '-')}');
       await dir.create(recursive: true);
       File('${dir.path}/session-abc.jsonl').writeAsStringSync(