When an issue bites: at build, at the first
message, or never.
This is what a host screening definitions before building needs in order
to quarantine one workflow rather than fail the whole engine. Not every code
check_workflow reports is a refusal, and the set that is has grown apart
from the set that is reported — Self::Advisory is the only class such a
host may safely ignore.
Unlike IssueCode, this is not#[non_exhaustive]. The axis is
when a definition goes wrong, and build time / first message / never
exhausts that by construction, so a host may match it and stay correct.