Throwaway branch for the GHA log-grouping trial matrix
(dev-docs/ci-test-log-grouping-design.md, verification item 3):
- 12 seeded failing tests in tests/smoke/trial-fail/ (hostile characters
  in messages to exercise escaping; 12 failures exercises the 9+1
  annotation cap and aggregate in the default path)
- default-run steps narrowed to an explicit 17-file subset (same
  single-process multi-file flow, minus full-suite runtime on the fork)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WFvZcK71eJ3gmmDT6RcFDd