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/golden/trail-rules.wml

@@ -0,0 +1,28 @@
+<?xml version="1.0"?>
+<!DOCTYPE wml PUBLIC "-//WAPFORUM//DTD WML 1.3//EN" "http://www.wapforum.org/DTD/wml13.dtd">
+<wml>
+<template>
+<do type="prev" label="Back"><prev/></do>
+</template>
+<card id="menu">
+<do type="prev"><noop/></do>
+<p>Live updates for the ridge trail network. Reception is spotty past the tree line, check before you go.</p>
+<p><a href="#card1">Weather</a></p>
+<p><a href="#card2">Trail Status</a></p>
+<p><a href="#card3">Contact</a></p>
+</card>
+<card id="card1" title="Weather">
+<p><b><big>Weather</big></b></p>
+<p>Cold and clear. Wind: <b>15 mph</b> gusting from the <i>northwest</i>. Permit fee: $$5.</p>
+</card>
+<card id="card2" title="Trail Status">
+<p><b><big>Trail Status</big></b></p>
+<p>- North Loop: open</p>
+<p>- South Loop: closed</p>
+<p>- Summit Spur: open, ice above 2000m</p>
+</card>
+<card id="card3" title="Contact">
+<p><b><big>Contact</big></b></p>
+<p>Ranger station: <a href="tel:+15555550123">call dispatch</a></p>
+</card>
+</wml>