Safety-review gate: discover BLOCK headings that depart from the required form #2182

Follow-up from the final review of #1992 (#1992 (comment)). P2: a current-head BLOCK can still be ignored when its heading departs from the required form (### heading, bold-text heading, or an em dash instead of the colon), combined with a bold head line or an edit, so an earlier APPROVE stays authoritative. Widen heading discovery so that any trusted, unfenced verdict-like comment naming the head fails validation unless it parses strictly. P3: ignore an APPROVE inside an HTML comment; document that a malformed review on an old head blocks the check until it is deleted (that fails closed).