[bug] Serialize run-parent directory creation across parallel lanes #502

FileExistsError: [Errno 17] File exists: 'runs'\n```\n\nAll pytest, Python quality gates, and the other smoke lanes succeeded.\n\n## Cause hypothesis\n\nThe implementation checks `directory.exists()` before `directory.mkdir(mode=0o700)`. Another lane can create the directory between those operations.\n\n## Impact\n\nA valid PR can fail the blocking full CI gate nondeterministically whenever parallel lanes share a missing run ancestor.\n\n## Acceptance\n\n- Make ancestor creation idempotent under concurrent creators without weakening the private-mode contract.\n- Add a deterministic focused regression test for the check/create race.\n- Verify the blocked PR against the fixed `main` SHA.\n\n## Lifecycle\n\n`DISCOVERED -> ENCODED -> ROUNDTRIP_VERIFIED -> BENCHMARKED -> REPORTED`; this defect blocks safe transition into benchmark execution and must fail closed rather than produce partial evidence.