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>

dart_test.yaml

@@ -10,3 +10,10 @@ tags:
   # under `dart test`. (Tag was historically named `forkpty`, before
   # T-96 replaced forkpty with posix_spawn.) See `test/pty/session_test.dart`.
   pty:
+  # Concurrency-vulnerable tests that flake in the parallel flutter pool
+  # (e.g. MessageBus delivery races). The parallel run excludes them
+  # (`--exclude-tags "pty || serial"`); ci/test.sh runs a separate
+  # `--tags serial --concurrency=1` pass and merges its coverage. Prefer a
+  # deterministic await first — only tag `serial` when isolation is the real
+  # fix. (T-193)
+  serial: