A merge whose base is neither main nor next now has an explicit,
gated path instead of forcing coordinator raw-API: --base-line <branch>
must MATCH the PR base (mismatch refuses naming both), the audit line
records the authorization, and queue guard / head pin / CI gates are
unchanged and still enforced (the suite proves the guard still runs
under the exception, stub rc propagates). Enrolled suite (3 arms).
Refusals stay exit 1: pr-merge's usage-class distinction remains
design-first work per the census (#1464 exclusions).
marcie