Make --- divide cards, and stop drawing ASCII rules

WML has no horizontal rule element -- its entire %layout entity is
`<br>` -- so a thematic break was being rendered as `<p>------------</p>`,
a hardcoded twelve-dash guess at screen width that wraps into nonsense on
a narrow device and is short on a wide one. It was the one construct
where the renderer invented a width out of nothing.

A `---` already looks like a divider and reads as one in every Markdown
document ever written, so it now is one: it starts a new card, by
default. Rules are never drawn, whether or not they divide.

Thematic breaks and `{.card Title}` markers are now a single mechanism --
an explicit divider, optionally carrying a title -- and compose freely in
one document. Both still win over --split-level. A section takes its
title from its own first heading when the divider does not supply one, so
the common shape needs no titles at all:

    Intro on the menu.

    ---

    ## Weather

    Cold and clear.

`--no-split-on-rule` / `split_on_rule: false` opts out of the splitting;
it does not bring the dashes back.

Three edges this opened, each fixed here rather than left to bite:

- Setext headings. `Heading` over `---` is an H2 in Markdown, but the
  parser was ATX-only, so it produced a paragraph plus a rule. Once a
  rule divides cards, a setext document would have split at every heading
  and stranded each heading as body copy on the card before it. Setext
  headings are now parsed, so such a document keeps its structure.
- Empty sections. A trailing `---`, or two in a row, produced an empty
  "Untitled" card and a menu entry leading to it. Sections with no
  renderable content are dropped.
- The multi-card threshold was two sections, so a document with a single
  divider collapsed back into one card. Any explicit divider now makes a
  multi-card deck: `A --- B` asks for two screens.

Also fixes a latent frontmatter bug this made far more likely: a document
opening with `---` and no `key: value` lines had its opening swallowed as
if the break were a frontmatter fence. A fenced block with no keys in it
is not frontmatter.

222 tests. Rendering examples/trail.md still matches md2txt's committed
output byte for byte; no golden file contains an ASCII rule any more.

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

tests/test_wml.py

@@ -71,12 +71,16 @@ class TestBlockMapping:
             ("Just text.\n", "<p>Just text.</p>"),
             ("- item\n", "<p>- item</p>"),
             ("> quoted\n", "<p><i>quoted</i></p>"),
-            ("---\n", "<p>------------</p>"),
         ],
     )
     def test_blocks(self, render, source, expected):
         assert expected in render(source)
 
+    def test_thematic_break_draws_nothing(self, render):
+        # WML has no rule element -- its whole %layout entity is <br> -- and
+        # a row of ASCII dashes is noise on a small screen.
+        assert "---" not in render("Before.\n\n---\n\nAfter.\n")
+
     def test_first_heading_is_emphasised_larger(self, render):
         markup = render("# Title\n\n## Sub\n")
         assert "<p><b><big>Title</big></b></p>" in markup