Fix parallel run-parent creation race

[Anionix]

Summary

• tolerate a concurrent creator only when it produced a real private directory
• fail closed for symlinks, non-directories, and group/world-accessible race winners
• deterministically simulate both safe and unsafe check/create races

Evidence

• GitHub trigger: PR Type adapter workload contracts #501 run 30074518996 failed only Smoke prompt lane with FileExistsError: runs; pytest and the other smoke lanes passed
• PYTHONPATH=src /Users/st/Documents/data-format-lab/.venv/bin/pytest tests/test_workflow.py -k 'missing_private_parent or missing_custom_ancestor or missing_default_parent' (6 passed)\n- uvx ruff check src/format_bench/workflow.py tests/test_workflow.py (passed)\n- uvx ty check --python /Users/st/Documents/data-format-lab/.venv src/format_bench/workflow.py reported only four pre-existing diagnostics on unchanged manifest boundaries; no diagnostic touches this diff\n- independent code-review skill: Standards 0 findings; Spec 0 findings at 269afe4a1a58c99180810ca35f52978efd39234f\n- NO_LEAN: no Lean sources or theorem boundary changed\n\nFixes [bug] Serialize run-parent directory creation across parallel lanes #502