Give frame titles as \frametitle so they are tagged as headings

Only \frametitle{...} produces a tagged heading in ltx-talk. Giving the
title as the argument to \begin{frame}{...} silently yields a frame with
no heading element at all, which is why the deck had sections but nothing
per slide. Use the command form throughout.

The resulting headings sit at ltx-talk's default H4, nested under the H1
that a section produces. That level can be remapped with \tagpdfsetup if
assistive software turns out to want frame titles higher up.

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

slides_template/example.tex

@@ -53,7 +53,8 @@
 \section{Introduction}
 \sectionpage
 
-\begin{frame}{A sample slide}
+\begin{frame}
+  \frametitle{A sample slide}
   A displayed formula:
   \[ \int_{-\infty}^\infty e^{-x^2} \, dx = \sqrt{\pi} \]
 
@@ -71,7 +72,8 @@
   \note<3->{Once the whole list is up, tie it back to the opening claim.}
 \end{frame}
 
-\begin{frame}{Short frame title}
+\begin{frame}
+  \frametitle{Short frame title}
   \e{A first} paragraph with \texttt{monospace text}.
 
   And then \e{a second} paragraph.