sync beads: math-mode (bd-w5ov closed) + 4 follow-ups

- bd-w5ov closed: math-mode rendering for HTML output (MathJax + KaTeX,
  CDN-default, native CLI verified live).
- bd-r7v2 closed: browser smoke for bd-w5ov (completed in same session).
- bd-si8b open: evaluate Temml as a third math engine for HTML output.
- bd-7sib open: add webtex/gladtex/mathml/plain math methods.
- bd-hva0 open: local-vendor opt-in for MathJax/KaTeX (offline rendering).
- bd-but3 open: hub-client math rendering blocked by iframe sandbox
  (discovered post-implementation; blocked on the separate service-worker
  iframe-isolation work).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Carlos Scheidegger