letter_template: add tagged PDF (PDF/UA-2) accessibility

Enables \DocumentMetadata (pdfversion=2.0, pdfstandard=ua-2) and loads
tagpdf with activate-all, plus hyperref metadata (pdftitle, pdfauthor,
pdflang, pdfdisplaydoctitle) since the letter class does not expose
\title/\author for the PDF management layer to pick up. Verified
compliant with verapdf's PDF/UA-2 profile.

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

letter_template/text.tex

@@ -1,3 +1,8 @@
+\DocumentMetadata{
+  pdfversion=2.0,
+  pdfstandard=ua-2,
+  lang=en-US,
+}
 \documentclass[12pt]{letter}
 
 \usepackage{fontspec}
@@ -6,6 +11,16 @@
 
 \usepackage[letterpaper,left=1.3in,right=1.3in,top=0.8in,bottom=0.8in]{geometry}
 
+\usepackage{tagpdf}
+\tagpdfsetup{activate-all}
+
+\usepackage{hyperref}
+\hypersetup{unicode=true,
+    pdftitle={Letter},
+    pdfauthor={Benjamin Mako Hill},
+    pdflang={en-US},
+    pdfdisplaydoctitle=true}
+
 \begin{document}
 
 \begin{letter}{Person Letter is Addressed To\\