Create custom run ancestors privately

[Anionix]

Fixes #498

Source review: #477 (comment)

Contract

• Walk from the nearest existing custom run-dir ancestor.
• Create every missing ancestor shallow-to-deep with requested mode 0700.
• Create the final run leaf separately with exclusive no-clobber semantics.
• Stop before the run leaf when ancestor creation fails.

Verification

• TDD red: intermediate ancestors were 0775/0777 under umask 002/000.
• Regression green: both collaborative-umask cases pass.
• Focused lifecycle tests: 7 passed.
• Ruff and git diff --check: PASS.
• Targeted correctness/scope review: PASS.
• Lean: NO_LEAN; directory modes and mkdir behavior are executable Python/OS contracts.

Exact SHA: 81b61f71dcfcd1ecaacc905cec22a9f0549decc5

Primary source:

• https://docs.python.org/3/library/pathlib.html#pathlib.Path.mkdir