emrg: the POSIX-arm guard treats any pytest.skip as a gate, so a which-only shell gate passed and test-windows went red #2348

What happened

tests/test_posix_arms_are_gated.py exists to catch an arm that reaches a POSIX-only primitive without a gate. On 2026-10-17 two new arms in tests/test_evolution_prompt_prompt_drift.py started a POSIX shell with a gate of

if shutil.which("bash") is None:
    pytest.skip("no bash on this host")

— the exact spelling the guard's own module docstring says is not the guard, because on windows-2025 a bare bash resolves to the WSL launcher C:\Windows\System32\bash.exe, which exists, cannot run a script, and answers "Windows Subsystem for Linux has no installed distributions."

The sweep called both arms GATED, CI agreed with the arms instead: test-windows failed on run 39027157421 (test_every_shell_block_parses, test_the_step_zero_line_finds_a_file_that_changed) while test passed — the one-leg red the file exists to prevent.

Why the guard missed it

_is_gated ends with

return "pytest.skip(" in _body_source(node)

so any pytest.skip(...) anywhere in a test body counts as a gate, whatever condition it sits under. The comment above it says a skip under no platform condition "is a red on both legs rather than the one-leg red this file exists for" — and that premise is false for exactly this shape, which is green on every POSIX host and red on the Windows leg.

Measurement, both directions

The guard's own _index over the head that went red (76cf63ad):

and with the fallback removed over the whole suite as it stands (after the arms were given a real gate): 0 arms lose their gate — nothing in this tree relies on that fallback, so removing it is not a rewrite of a hundred call sites.

Fix

Remove the fallback, so a pytest.skip counts only when its condition names the platform (a skipif condition, an if test, a module or class marker, or a module-level pytestmark), and add a control whose synthetic arm carries exactly the which("bash") is None skip and must be reported ungated. The arms themselves are fixed in #2343 — they now carry a marker that asks the shell to run one command (bash -c 'echo ok') before trusting the name.