Closes the 2 grep/Read-verified findings from the S12 cold full-brief re-review
(docs/remediation/review.md, WARN 0/1/1/0, 0 dropped) and closes the $-injection
CLASS — not the line — across the whole state-updater.mjs mutation surface.
See docs/remediation/review.md (S13 ALLOW, 0/0/0/0) for the full closure record:
replaceField -> replacement function; the 3 additive-insert sites -> functions
(m === $1, behavior-preserving); a scalar assert.match pins last_post_topic; and a
behavioral, coverage-complete, self-testing Section 12 guard (check-replace-safety.mjs)
that is mutation-proven. Docs three-doc + residuals updated. test-runner.sh 71/0/0,
node --test 98/98.
