fix(pdf): replace mr-pdf with custom puppeteer script using pdf-lib

mr-pdf v1.1.0 bundles Puppeteer v2.x with no --no-sandbox support,
causing a FATAL zygote_host_impl crash in GitHub Actions. Replace
with scripts/gen-pdf.js which:
- Uses the modern Puppeteer v23 already installed via mermaid-cli
- Launches Chrome with --no-sandbox --disable-setuid-sandbox
- Crawls all /turing/ pages via pagination-nav__link--next
- Hides sidebar/navbar/footer before printing each page
- Merges individual PDF buffers with pdf-lib into a single A4 PDF
- Prints progress and final page count

Remove mr-pdf dependency; add pdf-lib.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Alexandre Oliveira