templates: swap EB Garamond for Libertinus Serif

EB Garamond's bold weight ships with only 127 cmapped glyphs --- no
Latin Extended-A, no Vietnamese, no Greek, no Cyrillic, no fi/fl
ligatures --- so any bold text containing accented characters
(Polish, Czech, Vietnamese, etc., as well as bold author names in
citations) renders as tofu boxes or font-fallback substitutions.

Switch the default body font in paper_template, memo_template, and
letter_template to Libertinus Serif. It ships in texlive-fonts-extra,
has comprehensive Latin / Vietnamese / Greek / Cyrillic coverage in
every weight, and includes all the standard f-ligatures explicitly.

Each template also picks up a commented-out
\\setmainfont{Garamond Premier Pro} block plus a short note: Adobe
Garamond Premier Pro is Mako's preferred body font but is commercial
and not redistributable, so the public templates default to
Libertinus and CDSC members can swap in once they have the font
installed (e.g., in ~/.fonts/).

README updated to match.

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

README

@@ -47,8 +47,8 @@ On other systems, installing a full TeX Live distribution should provide
 everything needed.
 
 All four templates build with LuaLaTeX (via latexmk -lualatex). The text
-fonts are EB Garamond (provided by texlive-fonts-extra) and TeX Gyre
-Heros.
+fonts are Libertinus Serif and TeX Gyre Heros, both provided by
+texlive-fonts-extra.
 
 ================================
 === Accessibility ==============