ensure_comment's any-occurrence grep answers "have I ever said this", which
is right for a flag like blocked-unparseable and wrong for a value that
changes. A -> B -> A found A's own first echo and stayed silent, leaving the
thread's newest echo asserting B while the sweep gated on A: a stale parse
presented as the current one, and the third edit did change the parsed set,
so the criterion says it speaks.
blocked_parse_echo_needed compares this parse's marker against the LAST
blockers-parsed-* marker on the thread. The read stays inside guarded_read /
skip_issue, so an unreadable history still fails closed (#247 D1) rather than
answering "nothing echoed yet" and re-posting. ensure_comment is untouched
for every other caller.
Refs #252