fix: list-item {.x} line no longer terminates the list

[dereuromark]

Summary

Fixes a bug where a postfix {...} line inside a list item silently terminated the list, causing any following indented content (blockquote, paragraph continuation, nested list) to escape into a sibling paragraph outside the list.

New rule

A standalone {...} line on an indented continuation attaches to the <li> only when it is the last content line of the item. If another block follows within the same item, the {...} reverts to a standard djot block attribute for that following block; the list / item are not terminated.

Before / after

Input:

- item
  {.x}
  > quote

Before:

<ul>
<li class="x">
item
</li>
</ul>
<p>&gt; quote</p>

After:

<ul>
<li>
item
<blockquote class="x">
<p>quote</p>
</blockquote>
</li>
</ul>

Tests

Five new test methods in tests/TestCase/DjotConverterTest.php covering: blockquote-after-attr, paragraph-after-attr, nested-list-after-attr, last-line baseline regression guard, and loose-list (blank-line) regression guard.

Docs

Updated docs/reference/enhancements.md and docs/guide/syntax.md to document the rule with a worked example.

Related

Part of the attribute-hardening sweep. See sibling issue #185 tracking jgm/djot#262 (marker-adjacent syntax — out of scope here).

[codecov]

Codecov Report

❌ Patch coverage is 96.29630% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 91.11%. Comparing base (6b90dfe) to head (bf0b39c).

Files with missing lines	Patch %	Lines
src/Parser/BlockParser.php	96.29%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #186      +/-   ##
============================================
+ Coverage     91.10%   91.11%   +0.01%     
- Complexity     3321     3331      +10     
============================================
  Files            99       99              
  Lines          8497     8521      +24     
============================================
+ Hits           7741     7764      +23     
- Misses          756      757       +1     

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