The unguarded-pyarrow-import refusal is lost under pytest -n (xdist) #1305

Found by OffgridwithJD, reviewing #1286 at 3941f05.

What happens: pgc_vacuity refuses a test file that imports pyarrow outside expect.require_module. The scan runs in pytest_collect_file, and under xdist that hook runs in the workers. config.pgc_capability_imports never reaches the controller, so the refusal is never printed.

pytest        -> REFUSED  test_bad.py imports pyarrow outside require_module (line 1)   rc=1
pytest -n 2   -> 2 passed                                                                rc=0

(Measured with a stub pyarrow and a test file holding import pyarrow.parquet as pq.)

Impact: low. CI runs the pytest jobs serially (ci.yml:268 has no -n), so the gate holds there. A developer running -n gets no refusal. Nothing passes vacuously either way: without pyarrow the file errors at import, and with pyarrow the test really runs.

Possible fixes:

• Run the AST scan on the controller, over the collected paths.
• Or ship the sites back as a report property, the way pgc_declined already travels.

Done when: a pytester arm runs with -n 2 and sees the REFUSED line and rc=1, and that arm goes red against the current hook.

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