fix(testing): keep duration cache repo-relative

[lin-hongkuan]

Summary

• keep scripts/run_tests_parallel.py duration-cache writes limited to repo-local test files
• skip new timings for files outside the checkout, such as absolute temp probe paths
• prune stale POSIX and Windows absolute-path keys when rewriting test_durations.json

Testing

• ./venv/Scripts/python.exe -m pytest tests/test_run_tests_parallel.py::test_duration_cache_skips_paths_outside_repo tests/test_run_tests_parallel.py::test_duration_cache_drops_stale_absolute_keys -q
• ./venv/Scripts/python.exe -m pytest tests/test_run_tests_parallel.py -q
• ./venv/Scripts/python.exe scripts/check-windows-footguns.py --all

[teknium1]

Thanks for the focused cache-portability fix. Current main still writes duration keys via _format_file() in scripts/run_tests_parallel.py:500; that helper returns the absolute input after a failed relative_to() at scripts/run_tests_parallel.py:360-363. The existing external probe path in tests/test_run_tests_parallel.py:81-150 makes this reachable.

The PR correctly keeps display formatting unchanged while introducing a cache-specific repo-relative key, filters both POSIX and Windows absolute legacy keys, and adds direct coverage for both behaviors. GitHub currently reports the PR mergeable despite its old base.

Automated hermes-sweeper review.