Parallel Test A: DAG parallel execution

[maximilien]

This issue tracks Parallel Test A.\n\nTasks:\n1. Say 'PARALLEL A RUNNING'\n2. Write 'parallel-a-complete' to test-parallel-a.txt\n3. Say 'PARALLEL A COMPLETE'\n\nWe'll push a branch and open a PR.

[maximilien]

Progress update: Steps 1–3 completed. File committed in PR #28. Branch: test/parallel-a-20260612-091934.