Add BlockQuoteDivExtension (::: > fenced blockquote)

[dereuromark]

What

Adds an opt-in BlockQuoteDivExtension: a ::: div whose only token is > produces a semantic <blockquote>, the fenced analog of the existing ::: | line block.

::: >
Notes from the meeting:

- item one
- item two
:::

<blockquote>
<p>Notes from the meeting:</p>
<ul>
<li>
item one
</li>
<li>
item two
</li>
</ul>
</blockquote>

The verbose >-prefix form produces the identical output, but needs the marker on every line - the prose line, the blank separator, and each list item:

> Notes from the meeting:
>
> - item one
> - item two

The two are interchangeable; the fenced form just drops the per-line >. The gap grows with the content - a quote wrapping a fenced code block or a table needs a > on every one of those lines too.

Why

The >-prefix block quote is comfortable for prose, where lazy continuation lets you drop the marker on wrapped lines. It is not comfortable when the quote contains block structure (a list, a nested fence, a table), because lazy continuation folds only plain paragraph text into a quote, never new block structure. Every list item and nested block then needs its own >, which is tedious to write and error-prone to edit.

The fenced form removes the per-line marker while keeping identical semantics. It mirrors the language-neutral design of the ::: | line block: a single sigil on the opener, no English keyword, no per-line prefix. Distinct from ::: quote, which stays an <aside class="admonition quote"> rather than a semantic blockquote.

Behavior

• Output is identical to the >-prefix form: <blockquote>.
• Caption: a ^ attribution line right after the closing ::: wraps the quote in <figure>/<figcaption>, reusing the core caption handler.
• Attributes: a preceding {...} block attaches to the <blockquote> (consumed before the body is parsed, so an inner block cannot claim it; with a caption the attributes transfer onto the <figure>).
• Nesting: longer colon runs nest, per djot div semantics.

Design note

No core parser change. Because > is not a meaningful class, a block-pattern hook intercepts the ::: > opener, collects the inner lines with the standard div-closer detection, and parses them as block content into a BlockQuote node, exactly the property that let ::: | ship as an extension.

Docs

• docs/extensions/index.md - availability table row + a BlockQuoteDivExtension section.
• docs/guide/syntax.md - pointer from the block-quote section.
• docs/reference/enhancements.md - enhancement entry + feature-table row beside the line block.

[codecov]

Codecov Report

❌ Patch coverage is 97.87234% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.42%. Comparing base (58a181a) to head (ed67124).

Files with missing lines	Patch %	Lines
src/Extension/BlockQuoteDivExtension.php	97.87%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #250      +/-   ##
============================================
+ Coverage     92.37%   92.42%   +0.04%     
- Complexity     3576     3588      +12     
============================================
  Files           107      108       +1     
  Lines         10129    10176      +47     
============================================
+ Hits           9357     9405      +48     
+ Misses          772      771       -1     

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