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/README.ltx-talk

@@ -98,10 +98,21 @@ Upstream: https://github.com/mrc-pop/trantor
 === Known gaps =================
 ================================
 
-Frame titles are not headings. ltx-talk's role map declares
-frametitle -> H4, but no such elements are emitted, so the tag tree has
-sections but no per-slide headings. The manual calls the relationship
-between frame titles and sectioning "still very much open".
+Frame titles must be given as \frametitle{...} inside the frame, not as
+the argument to \begin{frame}{...}. Only the command form is tagged; the
+argument form produces a frame with no heading at all, silently. This
+looks like an upstream oversight rather than a decision, so it may change.
+
+ltx-talk tags frame titles at H4, deliberately well below sections and
+subsections, and the manual describes the relationship between frame
+titles and sectioning as "still very much open". That default is left
+alone here. It can be changed with
+
+  \tagpdfsetup{role/new-tag = frametitle/H1}
+
+but H1 would put frame titles at the same level as the section headings
+that contain them, so it is worth deciding what an accessibility checker
+should see before reaching for it.
 
 Overlay tagging has an open upstream bug, issue #33, where \only<1> and
 \only<2> produce output that fails validation. The \pause and <+-> forms