Phase 12: LEHRUNTERLAGE + HTML rendering + i18n fix + doc-standard retrofit

- LEHRUNTERLAGE.md: Phase-12 destillation of the project's protocol, with
  11 embedded screenshots and links to the F/T-contracts and protocol.
- build_html.py + 5 generated .html files: self-contained, GitHub-style
  heading slugs, MD→HTML link rewriting, chess-themed embedded CSS.
- Tests + Teaching-guide footer links in index.html, i18n in en+de.
- i18n status text now refreshes on language switch: setStatus accepts
  [key, ...args] specs, stored and re-rendered in applyI18n().
- T_CONTRACT Section 8: minimum code-documentation standard (module
  header, function headers 1-2 lines minimum, inline notes at non-obvious
  points, English-only). No-TS clarification. 250-LOC rule softened to
  a guideline excluding comment lines.
- Code retrofit: ~43 missing function/method headers added across all 9
  JS modules; inline notes deepened on solver._init / _pickCandidates
  and state._loadFromHash / _saveToHash.

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