Review follow-ups: serve text/html sibling assets under the artifact sandbox CSP (direct navigation no longer bypasses it); resolve MIME from the request name first with target extension as fallback; make security fixtures cross-platform (no angle brackets in filenames, skip when symlink creation is denied). Residual check-then-read TOCTOU documented as accepted for the loopback-local threat model. Tests 33/33 GREEN, ESLint clean.