perf(scripts): single-parse website build + CI caching (#144)

Optimize the lesson -> GitHub Pages conversion in build_website.py:

- BS4 single parse: thread one BeautifulSoup tree through heading-id
  normalisation, TOC extraction, and link rewriting instead of
  reparsing the rendered HTML 3x per page. Build drops 2.97s -> 1.77s
  (~40% faster) on the ~222-page site; html.parser reparsing was the
  dominant cost.
- Nav O(n^2) -> O(n): build the sidebar section grouping once
  (build_nav_skeleton) and resolve per-page relative URLs in localize_nav,
  removing ~49k redundant relative_link/grouping calls.
- Read each .md once: cache source text on PageInfo.content and reuse it
  across collect_pages, render_pages, and copy_assets (was 2-3 reads/page).
- CI: enable uv cache and cache scripts/.vendor-cache so deploys skip cold
  dep resolution and vendor (Tailwind/Mermaid/fonts) downloads.

Public string APIs (derive_page_title, render_markdown, extract_toc,
rewrite_links, normalise_heading_ids) preserved as thin wrappers over the
new soup-based cores. Full suite green (92 tests), ruff/mypy clean.

Output is semantically identical to the prior build; the only diff is raw
hand-written <img/> badges serializing as <img></img> (valid HTML5,
renders identically; nothing validates generated site HTML).

.github/workflows/pages.yml

@@ -30,6 +30,16 @@ jobs:
 
       - name: Install uv
         uses: astral-sh/setup-uv@v4
+        with:
+          enable-cache: true
+
+      - name: Cache vendor assets
+        uses: actions/cache@v4
+        with:
+          path: scripts/.vendor-cache
+          key: vendor-${{ hashFiles('scripts/vendor_assets.py') }}
+          restore-keys: |
+            vendor-
 
       - name: Build website
         run: uv run scripts/build_website.py --verbose