Add : + continuation marker for multiple dd elements

[dereuromark]

Summary

Adds explicit syntax for creating multiple <dd> elements for the same term in definition lists while maintaining djot spec compliance.

Demo: https://sandbox.dereuromark.de/sandbox/djot?d=OiBEam90CiAoL2TKksmRdC8pCiAgQSBsaWdodHdlaWdodCBtYXJrdXAgbGFuZ3VhZ2UgZGVzaWduZWQgYnkgSm9obiBNYWNGYXJsYW5lLAogIGNyZWF0b3Igb2YgUGFuZG9jIGFuZCBDb21tb25NYXJrLgo6ICsKICBFbmhhbmNlZCBieSBNYXJrIFNjaGVyZXIgYW5kIERqb3QgUEhQIGltcGxlbWVudGF0aW9uIHRvIHN1cHBvcnQgbW9yZSBhd2Vzb21lbmVzcy4KCjogTWFya2Rvd24KOiBDb21tb25NYXJrCjogR0ZNCgogIEVhcmxpZXIgbWFya3VwIGxhbmd1YWdlcyB0aGF0IGluc3BpcmVkIERqb3QncyBkZXNpZ24sCiAgYnV0IHdpdGggbGVzcyBjb25zaXN0ZW50IHBhcnNpbmcgcnVsZXMuCgogIFRoZXkgY2FuIGJlIHVwIHRvIHR3byB0aW1lcyBzbG93ZXIu

The Problem

HTML definition lists support multiple <dd> elements per term (for multiple distinct definitions), but the djot spec uses blank lines for paragraph separation within a single <dd>. There was no way to create multiple <dd> elements.

The Solution

• Spec compliant: Blank lines within definition content create paragraphs in the same <dd>
• New syntax: : + marker explicitly creates additional <dd> elements

Example

: Term

  First paragraph

  Second paragraph (same dd - multiple paragraphs)

: +

  Third paragraph (new dd - separate definition)

Produces:

<dl>
<dt>Term</dt>
<dd>
<p>First paragraph</p>
<p>Second paragraph</p>
</dd>
<dd>
<p>Third paragraph</p>
</dd>
</dl>

Roundtrip Support

• HtmlToDjot converter now outputs : + markers for multiple <dd> elements
• Full roundtrip fidelity: HTML → Djot → HTML preserves structure

Refs jgm/djot#128

[codecov]

Codecov Report

❌ Patch coverage is 95.65217% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.38%. Comparing base (2946eb7) to head (7d1c3e0).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Parser/BlockParser.php	95.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #49      +/-   ##
============================================
- Coverage     93.72%   93.38%   -0.34%     
+ Complexity     1928     1924       -4     
============================================
  Files            74       74              
  Lines          5177     5173       -4     
============================================
- Hits           4852     4831      -21     
- Misses          325      342      +17     

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