pr-review-shadow records REFUSE and exits 0 on the PR path (banned third state, C324) #5996

pr-review-shadow records REFUSE on the PR path and exits 0 (report-only third state)

On the PR path, the pr-review-shadow step in x86-main records a REFUSE verdict and still exits 0. That is a report-only third state: it can neither block nor pass. C324 bans a third state on a PR or release path (a check is a GATE that can block, or LAB that never runs there).

Found by the quorum on #5994 (receipt: handoff/triage-quorum-5994-gate-2016Z.md, Haiku lane). Pre-existing in x86-main, not introduced by #5994 (PMAT-5955).

Acceptance

• Either REFUSE exits non-zero on the PR path (it is a GATE), or the step moves to a nightly LAB lane and is removed from PR/merge-queue jobs.
• A self-test case: REFUSE -> non-zero (or step absent from the PR path).

Owner: wsweep, after #5994 v8. Gate content change: needs non-author seats.