forgeVerdict(view, now) names what is wrong with the forge itself, in one pure function #2323

First half of #2284, which failed twice because it changed the /healthz shape (test/vitals.test.ts lists its keys exactly) and the board's ask count (test/review-wait.test.ts).

Done looks like:

• [ ] A pure function next to src/kernel/guard.ts takes the guard view and the time. It returns undefined, or a short text built from the incident's own message. It reports a held or stuck incident, an oom:* incident, draining for more than 5 minutes, or a stale view. A view that has never been written is not stale.
• [ ] Unit tests: a held incident and, separately, a stale view each give a text. An empty view and a never-written view give undefined.
• [ ] The /healthz shape and the board stay as they are. Run test/vitals.test.ts and test/review-wait.test.ts and quote the result.

Parked by the crew: the check failed ("mail over SMTP, TLS > a certificate that verifies but names another host is refused before AUTH") and the decider saw no point in another try (pull #2325 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.