memo, letter: enable tagging with \DocumentMetadata{tagging=on}

These templates declared pdfstandard=ua-2 and loaded tagpdf with
\tagpdfsetup{activate-all}. That combination sets /Marked true and
writes the PDF/UA-2 claim into the metadata, but activate-all is
tagpdf's low-level switch and does not turn on the kernel code that
tags document structure. The result was a file announcing itself as
accessible while carrying nothing for assistive software to read: the
memo's structure tree held one /Document and two /Link nodes, and the
letter's held /Document alone.

tagging=on loads that kernel code. The memo now tags three headings, a
subsection, its paragraphs and its bibliography list; the letter tags
all ten body paragraphs. The manual tagpdf lines are no longer needed
since the kernel loads it.

This requires TeX Live 2025 or later; on TeX Live 2024 the key is
unknown and the build stops.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

letter_template/text.tex

@@ -1,4 +1,14 @@
+% Tagged PDF (PDF/UA-2) accessibility setup.
+%
+% tagging=on is what actually builds the structure tree: it loads the
+% kernel code that tags paragraphs, headings, lists, tables, and
+% floats. Declaring pdfstandard=ua-2 on its own only writes the claim
+% into the metadata; without tagging=on the PDF is marked as UA-2 but
+% carries no structure for a screen reader to read. This needs TeX
+% Live 2025 or later --- on Overleaf, set it in Menu > Settings >
+% TeX Live version.
 \DocumentMetadata{
+  tagging=on,
   pdfversion=2.0,
   pdfstandard=ua-2,
   lang=en-US,
@@ -23,9 +33,6 @@
 
 \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},