Add tagged PDF accessibility support

Switch to LuaLaTeX (better tagged PDF support), add \DocumentMetadata
for PDF 2.0 and language metadata, enable tagpdf tagging, and mark
tikz overlay commands (\credit, \marktopleft, \markbottomright) as
PDF artifacts.

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

slides_template/example.tex

@@ -1,8 +1,15 @@
+\DocumentMetadata{
+  pdfversion=2.0,
+  lang=en-US,
+}
 \documentclass[10pt,xcolor=dvipsnames,aspectratio=169]{beamer}
 
 \usetheme{metropolis}
 \usepackage{appendixnumberbeamer}
 
+\usepackage{tagpdf}
+\tagpdfsetup{activate-all}
+
 \usepackage{pgfpages}
 \input{notes.config}
 
@@ -109,10 +116,10 @@ BoldItalicFont  =   *-BoldItalic
 \tikzstyle{every picture}+=[overlay,remember picture]
 
 % add functions to circle parts of slides (e.g., in tables)
-\newcommand\marktopleft[1]{\tikz \node (marker-#1-a) at (0,1.5ex) {};}
+\newcommand\marktopleft[1]{\tagmcbegin{artifact}\tikz \node (marker-#1-a) at (0,1.5ex) {};\tagmcend}
 \newcommand\markbottomright[1]{%
-  \tikz{\node (marker-#1-b) at (0,0) {};}
-  \tikz[dashed,inner sep=3pt]{\node[violet!75,ultra thick,draw,rounded rectangle,fit=(marker-#1-a.center) (marker-#1-b.center)] {};}}
+  \tagmcbegin{artifact}\tikz{\node (marker-#1-b) at (0,0) {};}\tagmcend
+  \tagmcbegin{artifact}\tikz[dashed,inner sep=3pt]{\node[violet!75,ultra thick,draw,rounded rectangle,fit=(marker-#1-a.center) (marker-#1-b.center)] {};}\tagmcend}
 
 % create an empty quotetxt so we can reuse it
 \newcommand{\quotetxt}{}
@@ -153,7 +160,7 @@ BoldItalicFont  =   *-BoldItalic
 \newcommand{\credit}[1]{%
   \tikz[overlay]{\node at (current page.south west)
     [anchor=south west, yshift=3pt, xshift=2pt]
-    {\smaller \smaller {[}#1{]}};}}
+    {\tagmcbegin{artifact}\smaller \smaller {[}#1{]}\tagmcend};}}
 
 \setlength{\parsep}{1em}