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_config.py

@@ -82,3 +82,15 @@ class TestValidation:
     def test_non_boolean_menu_in_frontmatter(self):
         with pytest.raises(ConfigError, match="menu must be a boolean"):
             DeckConfig.resolve({"menu": "sometimes"})
+
+
+class TestSplitOnRule:
+    def test_defaults_to_on(self):
+        assert DeckConfig.resolve().split_on_rule is True
+
+    def test_frontmatter_can_disable(self):
+        assert DeckConfig.resolve({"split_on_rule": "false"}).split_on_rule is False
+
+    def test_flag_overrides_frontmatter(self):
+        config = DeckConfig.resolve({"split_on_rule": "true"}, {"split_on_rule": False})
+        assert config.split_on_rule is False