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_deck.py

@@ -211,3 +211,108 @@ class TestSlugify:
         assert slugify("", "card1") == "card1"
         assert slugify(None, "card1") == "card1"
         assert slugify("!!!", "card1") == "card1"
+
+
+TWO_BY_RULE = """\
+Intro text.
+
+---
+
+## Weather
+
+Cold and clear.
+
+---
+
+## Status
+
+North Loop open.
+"""
+
+
+class TestRuleDividers:
+    """A thematic break divides cards; WML has no rule element to draw."""
+
+    def test_rules_divide_by_default(self, render):
+        assert card_ids(render(TWO_BY_RULE)) == ["menu", "card1", "card2"]
+
+    def test_sections_take_their_own_heading_as_title(self, render):
+        # A rule carries no title, so the section's first heading supplies it.
+        markup = render(TWO_BY_RULE)
+        assert '<card id="card1" title="Weather">' in markup
+        assert '<card id="card2" title="Status">' in markup
+
+    def test_a_single_rule_makes_two_cards(self, render):
+        # Regression: the multi-card threshold used to be two sections, so
+        # one divider collapsed back into a single card.
+        assert card_ids(render("Intro.\n\n---\n\n## Alpha\n\nBody.\n")) == [
+            "menu",
+            "card1",
+        ]
+
+    def test_no_rule_is_ever_drawn(self, render):
+        assert "------" not in render(TWO_BY_RULE)
+
+    def test_disabling_keeps_one_card_and_still_draws_nothing(self, render):
+        markup = render(TWO_BY_RULE, split_on_rule=False)
+        assert card_ids(markup) == ["card1"]
+        assert "------" not in markup
+
+    @pytest.mark.parametrize("glyph", ["---", "***", "___", "- - -", "*****"])
+    def test_every_thematic_break_glyph_divides(self, render, glyph):
+        markup = render(f"Intro.\n\n{glyph}\n\n## Alpha\n\nBody.\n")
+        assert card_ids(markup) == ["menu", "card1"]
+
+    def test_rules_and_card_markers_compose(self, render):
+        markup = render("Intro.\n\n---\n## A\nx\n\n{.card Named}\ny\n")
+        assert card_ids(markup) == ["menu", "card1", "card2"]
+        assert '<card id="card2" title="Named">' in markup
+
+    def test_explicit_dividers_win_over_split_level(self, render):
+        markup = render("Intro.\n\n---\n\n## A\n\n## B\n", split_level=2)
+        assert card_ids(markup) == ["menu", "card1"]
+
+
+class TestEmptySections:
+    def test_trailing_rule_adds_no_empty_card(self, render):
+        # Ending a document with `---` is common and must not produce an
+        # "Untitled" card with a menu entry leading nowhere.
+        markup = render("Intro.\n\n---\n\n## A\n\nBody.\n\n---\n")
+        assert card_ids(markup) == ["menu", "card1"]
+        assert "Untitled" not in markup
+
+    def test_consecutive_rules_collapse(self, render):
+        markup = render("Intro.\n\n---\n\n---\n\n## A\n\nBody.\n")
+        assert card_ids(markup) == ["menu", "card1"]
+
+    def test_leading_rule_is_harmless(self, render):
+        markup = render("---\n\n## A\n\nBody.\n")
+        assert "Untitled" not in markup
+
+    def test_document_of_only_a_rule(self, render):
+        assert card_ids(render("---\n")) == ["card1"]
+
+
+class TestSetextHeadings:
+    """`---` under text is an H2, not a divider -- Markdown says so, and
+    getting it wrong would split at every heading of a setext document."""
+
+    def test_dashes_under_text_make_a_heading_not_a_split(self, render):
+        markup = render("My Heading\n---\n\nBody text.\n")
+        assert card_ids(markup) == ["card1"]
+        assert "<p><b><big>My Heading</big></b></p>" in markup
+
+    def test_equals_make_a_level_one_heading(self, render):
+        assert "<p><b><big>Big Title</big></b></p>" in render("Big Title\n===\n\nBody.\n")
+
+    def test_setext_heading_can_drive_split_level(self, render):
+        markup = render("# Top\n\nIntro.\n\nAlpha\n-----\n\nA.\n\nBeta\n----\n\nB.\n", split_level=2)
+        assert card_ids(markup) == ["menu", "card1", "card2"]
+
+    def test_a_rule_after_a_blank_line_is_still_a_rule(self, render):
+        # The paragraph has been flushed, so this underlines nothing.
+        assert card_ids(render("Text.\n\n---\n\n## A\n\nBody.\n")) == ["menu", "card1"]
+
+    def test_two_dashes_underline_but_do_not_divide(self, render):
+        # `--` is a valid setext underline but not a thematic break.
+        assert card_ids(render("Heading\n--\n\nBody.\n")) == ["card1"]