List continuation marker: attach a block to a list item with +

[dereuromark]

Draft / proposal / demo. Always-on. A djot-php addition (not canonical djot — djot.js has no + continuation marker). The official djot test suite stays green. Companion to #227 (compact list blocks); this one is the explicit, no-indent variant.

What it does

A lone + on its own line, at the list-marker column, attaches the following block to the current list item — no blank line, list stays tight, and the block body is not indented. (AsciiDoc's list-continuation idiom.)

- Build the image
+
```sh
docker build -t app .

• Push it

→
```html
<ul>
<li>
Build the image
<pre><code class="language-sh">docker build -t app .
</code></pre>
</li>
<li>
Push it
</li>
</ul>

A quote, table or ::: admonition attaches the same way:

- item
+
> a note attached to the item
- next

→ <li>\nitem\n<blockquote><p>a note attached to the item</p></blockquote>\n</li> + <li>\nnext\n</li>.

Why — where it beats plain indentation

#227 already lets an indented block stay tight. + adds the no-indent case, which matters when indentation is painful:

• Pasting code — no re-indenting every line by the marker width.
• Deep / wide markers — under 10. or three levels deep, the required indent is 4–8 spaces; + keeps the block flush-left.

It adds no new structure (same result is reachable by indenting under the item); it's authoring sugar for the flush-left case.

Collision safety

+ is a bullet marker — but only + + content is a bullet. A bare + is not, so:

• inside a +-bulleted list it's unambiguous (not an empty sibling item);
• outside any list a lone + stays ordinary paragraph text (<p>+</p>).

Both pinned in tests.

Implementation

• The item-content collector stops at a bare + (instead of swallowing it as lazy continuation); the list loop then attaches the following flush-left block(s) to the last item and keeps the list tight.
• One block per +, up to the next blank line / sibling item / further +.

Tests & docs

• ListContinuationMarkerTest — attach code/quote, no-loosen, bullet + outside-list non-collision.
• Full suite 2539 green, official djot suite 250/250.
• Docs: new "List Continuation Marker" section in guide/syntax.md, flagged as a djot-php addition.

Status / relationship to #227

Independent branch from master. #227 (compact list blocks) is the zero-syntax win for the common case; this PR is the explicit + variant for non-indented blocks. They can land independently or together. Draft for discussion on whether the + sigil is worth carrying in a djot port vs leaving it to dialects.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.91%. Comparing base (d4de671) to head (d72b4ce).

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #228      +/-   ##
============================================
+ Coverage     91.87%   91.91%   +0.04%     
- Complexity     3512     3538      +26     
============================================
  Files           105      105              
  Lines          9944     9995      +51     
============================================
+ Hits           9136     9187      +51     
  Misses          808      808              

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[dereuromark]

Moving to https://github.com/markup-carve/carve for the + bullet point operator collision here (and upstream denied).