Four origin/main merges brought back `linux_only` / `macos_only` /
`windows_only` marks in 41 test files, along with the pre-platforms()
versions of scripts/ci/list_os_marked_tests.py and check_os_marker_fakes.py.
Because the legacy names are no longer registered, pytest treated them as
unknown marks — a warning — so every Windows- or macOS-only test RAN on
Linux (test_local_runtime_recovery.py tripped the live-system kill guard).
Rewrite the marks, restore the platforms()-aware CI scripts (keeping main's
os.walk fix for vanishing __pycache__ dirs), drop the stale _BASELINE entries,
and make the conftest reject the retired marks outright so the next merge
cannot resurrect them silently.
071ccfbcdf