rodboev. Default-off; OFF path byte-identical to master (cache key 'u' + _renderUserFencedBlocks
unchanged when window._renderUserMarkdown falsy). ON routes user text through the same renderMd
SAFE_TAGS/_isSafeUrl sanitizer as assistant msgs. Nathan override-approved (likes the idea, opt-in).
Co-authored-by: rodboev <rodboev@users.noreply.github.com>
