fix(make-pdf): Liberation Sans font fallback for Linux rendering

On Linux (Docker, CI, servers), neither Helvetica nor Arial exist. Our CSS
stacks were falling through to DejaVu Sans — wider letterforms that look like
Verdana, not the intended Helvetica/Faber look. Liberation Sans is the standard
metric-compatible Arial clone (SIL OFL 1.1, apt package fonts-liberation).

- print-css.ts: all four font stacks (body + @top-center + @bottom-center +
  @bottom-right CONFIDENTIAL) gain "Liberation Sans" between Helvetica and
  Arial. File-header docblock updated to reflect the new stack.
- .github/docker/Dockerfile.ci: explicit apt-get install fonts-liberation +
  fontconfig with retry, fc-cache -f, and a verify step that fails the build
  loud if the font disappears. Playwright's install-deps happens to pull this
  in today but the dep is implicit and could silently regress.
- SKILL.md.tmpl: one-sentence note pointing Linux users at fonts-liberation.
- SKILL.md: regenerated via bun run gen:skill-docs --host all (only make-pdf's
  generated file changed — verified clean diff scope).
- render.test.ts: 2 assertions — Liberation Sans in body stack AND in at least
  one @page margin-box rule (proves all four intended stacks got touched, not
  just one).

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

make-pdf/SKILL.md

@@ -470,6 +470,10 @@ left-aligned body, Helvetica throughout, curly quotes and em dashes, optional
 cover page and clickable TOC, diagonal DRAFT watermark when you need it.
 Copy-paste from the PDF produces clean words, never "S a i l i n g".
 
+On Linux, install `fonts-liberation` for correct rendering — Helvetica and Arial
+aren't present by default, and Liberation Sans is the standard metric-compatible
+fallback. CI and Docker builds install it automatically via Dockerfile.ci.
+
 ## MAKE-PDF SETUP (run this check BEFORE any make-pdf command)
 
 ```bash