README: document lualatex requirement and accessibility status

Adds texlive-luatex to the Debian dependency list, documents the
switch to LuaLaTeX + EB Garamond/TeX Gyre Heros, and adds a new
Accessibility section describing which templates produce tagged
PDF/UA-2 output and why the memoir-based templates don't (yet).

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

README

@@ -33,20 +33,38 @@ git format-patch to generate patch files to send by email:
 On Debian or Ubuntu, install the following packages:
 
   * latexmk
+  * texlive-luatex
   * texlive-latex-recommended
   * texlive-fonts-extra
   * texlive-fonts-recommended
   * texlive-bibtex-extra
 
-  apt install latexmk texlive-latex-recommended \
+  apt install latexmk texlive-luatex texlive-latex-recommended \
   texlive-fonts-extra texlive-fonts-recommended \
   texlive-bibtex-extra
 
 On other systems, installing a full TeX Live distribution should provide
 everything needed.
 
-The templates make extensive use of the URW Garamond font, available from
-CTAN: https://ctan.org/pkg/urw-garamond
+All four templates build with LuaLaTeX (via latexmk -lualatex). The text
+fonts are EB Garamond (provided by texlive-fonts-extra) and TeX Gyre
+Heros.
+
+================================
+=== Accessibility ==============
+================================
+
+The letter and poster templates produce PDF/UA-2 compliant tagged PDFs
+via the LaTeX tagging project (\DocumentMetadata + tagpdf). You can
+verify compliance with verapdf: https://verapdf.org/
+
+The paper and assignment templates are based on the memoir class, which
+is not yet compatible with the LaTeX tagged-PDF code. They load the
+pdfmanagement-testphase layer for PDF metadata only. When memoir gains
+tagging support upstream, the commented-out \DocumentMetadata and tagpdf
+lines in each template can be re-enabled. See:
+
+  https://github.com/latex3/tagging-project/issues/910
 
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 === Branches ===================