feat: attach attributes to list items via marker-adjacent syntax (jgm/djot#262)

[dereuromark]

Summary

Implements the djot proposal jgm/djot#262: attributes in curly braces that immediately follow a list marker (no space before the brace) attach to the <li> element.

+{.blue} A blue list item.
+{#id1 .highlight} Item with id and class.
1.{data-value="test"} Numbered item with a data attribute.

renders as:

<ul>
<li class="blue">A blue list item.</li>
<li id="id1" class="highlight">Item with id and class.</li>
</ul>
<ol>
<li data-value="test">Numbered item with a data attribute.</li>
</ol>

Details

• Works across every marker type: bullet (-/*/+), ordered numeric (1./1)), parenthesized ((1)/(a)), roman, alpha, and task lists.
• Adjacency rule: a space before the brace changes the meaning. + {.blue} keeps the brace as item content (a block attribute for the following block inside the item), it does not attach to the <li>.
• The older separate-indented-line form ({...} on its own line under the item) is soft-deprecated but still attaches to the <li> for back-compat. Marker attributes seed the item; the separate-line form merges on top.
• The opt-in blocksInterruptParagraphs mode now recognizes the attributed markers, so -{.x} item / 1.{.x} item interrupt an open paragraph exactly like their plain counterparts.

Implementation

• ListParser::parseListItemMarker() captures an optional {...} block immediately after each marker and returns it in a new attrs key.
• BlockParser seeds the list item's attributes from attrs, and excludes the attribute width from the ordered-list content-indent column so multi-line attributed items keep the correct continuation indent.
• The letter-marker fast-path gate and the paragraph-interruption detector both accept the optional adjacent {...}.

Docs

docs/guide/syntax.md now leads with the marker-adjacent syntax and marks the separate-line form as soft-deprecated.

[codecov]

Codecov Report

❌ Patch coverage is 98.07692% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 91.89%. Comparing base (f80b8da) to head (bbeb295).

Files with missing lines	Patch %	Lines
src/Parser/BlockParser.php	93.75%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #241   +/-   ##
=========================================
  Coverage     91.89%   91.89%           
- Complexity     3527     3532    +5     
=========================================
  Files           106      106           
  Lines          9970     9985   +15     
=========================================
+ Hits           9162     9176   +14     
- Misses          808      809    +1     

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