Thirty assertions over the change shapes the filters have to get right,
including the ones that only appeared under review: a large diff that used to
lose flags to SIGPIPE, renames that hide their source path, and an unresolvable
base ref that must fail closed rather than emit every flag as false.
It builds a throwaway repo per case, so it needs no fixtures and leaves nothing
behind. Nothing runs it in CI yet; it is a development tool for whoever changes
a regex next.
