Add LineBlockDivExtension (::: | fenced line blocks)

[dereuromark]

Summary

Adds LineBlockDivExtension: a fenced line block written as a ::: div whose only class token is a pipe - ::: |. Produces the same line-block div as the existing |-prefixed form, without prefixing every line.

::: |
The limerick packs laughs anatomical
  Into space that is quite economical.

But the good ones I've seen
  So seldom are clean
:::

Inside the fence: soft line breaks become hard breaks (<br>), leading whitespace is preserved, a blank line separates stanzas (each becomes its own paragraph), and inline djot still parses normally.

Indentation preserved in every format

Leading whitespace is kept as a non-breaking space, so the indent survives without any CSS - and it composes with nesting. An indented poem inside a list item:

- A poem in a list:

  ::: |
  Roses are red,
    Violets are blue,
    Sugar is sweet,
  And so are you.
  :::

renders (HTML):

<ul>
<li>
<p>A poem in a list:</p>
<div class="line-block">
<p>Roses are red,<br>
&nbsp;&nbsp;Violets are blue,<br>
&nbsp;&nbsp;Sugar is sweet,<br>
And so are you.</p>
</div>
</li>
</ul>

Per format:   in HTML, a real non-breaking space (U+00A0) in Markdown - which keeps the indent through a round-trip re-render and never trips the indented-code-block rule - and an ordinary space in plain-text / ANSI. Tabs expand to four-column stops.

Why

Per djot#29: a leading | on every line (Pandoc-style) can be confused with pipe tables and is awkward to edit; an English-keyword div class (::: verse) was undesirable. A language-neutral | marker on the div opener sidesteps both. The pipe is consumed as the marker, so the output is a line-block div, never a literal class="|" - and because | is not a meaningful class, intercepting it cannot collide with real usage.

Design

• Parsing is a pure extension. Registered via addBlockPattern, which runs before the core div parser, so ::: | is intercepted before it could become a class="|" div. Reuses the core LineBlock node and the core FencedBlockParser detectors so code-fence/closer recognition matches the built-in div parser. Composes with blockquotes and list items.
• Indentation uses the existing internal non-breaking-space placeholder (U+E000) - a private-use sentinel, so it never collides with a literal U+00A0 in the author's text. The HTML escaper already turns it into  ; the non-HTML renderers convert it as their final output step (Markdown -> U+00A0, plain/ANSI -> space). This also fixes a pre-existing case where the escaped-space placeholder leaked into non-HTML output.
• Opt-in: $converter->addExtension(new LineBlockDivExtension());

Docs

• docs/extensions/index.md: new LineBlockDivExtension section + table entry.
• docs/guide/syntax.md: cross-reference from the Line Blocks section.

Notes

Draft - proposal-stage feature tracking djot#29. Class name settled as line-block; stanza handling (blank line = separate paragraph) open to review.

[codecov]

Codecov Report

❌ Patch coverage is 98.95833% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.04%. Comparing base (9a51f5a) to head (7f69890).

Files with missing lines	Patch %	Lines
src/Extension/LineBlockDivExtension.php	98.88%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #243      +/-   ##
============================================
+ Coverage     91.89%   92.04%   +0.14%     
- Complexity     3532     3559      +27     
============================================
  Files           106      107       +1     
  Lines          9985    10078      +93     
============================================
+ Hits           9176     9276     +100     
+ Misses          809      802       -7     

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[dereuromark]

Merging without release for demo in sandbox for us for now.

Net-side effect of fixing the NBSP transformation affects all renderers => positive