Preserve medial gaps in ::: | line blocks

[dereuromark]

Summary

Follow-up to #243. The LineBlockDivExtension preserved only leading indentation; medial and trailing runs of two or more spaces were collapsed by normal inline parsing - so a verse lost the alignment gaps it depends on.

This is the inconsistency Omikhleia raised on djot#29: a line block whose whole point is significant whitespace should keep the medial gap too, not just the leading indent. The canonical example is the caesura of Old English verse:

::: |
Hwaet! We Gardena    in geardagum
theodcyninga       thrym gefrunon
:::

renders (HTML) with the gaps intact:

<div class="line-block">
<p>Hwaet! We Gardena&nbsp;&nbsp;&nbsp;&nbsp;in geardagum<br>
theodcyninga&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;thrym gefrunon</p>
</div>

Rule

• Leading whitespace (indentation): always preserved, even a single space (unchanged).
• Inner / trailing runs of two or more columns (medial gaps, inline alignment): now preserved.
• A single inner space stays an ordinary, collapsible space, so a long line can still wrap between words.

This matches Pandoc's line-block definition (every space the author writes is significant) while keeping ordinary word spacing wrappable.

Implementation

• Replaces splitLeadingWhitespace with a single appendLine pass that walks each line, emitting preserved columns via the existing internal non-breaking-space placeholder (U+E000) and inline-parsing the text segments between gaps.
• Per format:   in HTML, real U+00A0 in Markdown (survives round-trip, never read as indented code), ordinary space in plain text / ANSI. Tabs expand to four-column stops.

Docs

• docs/extensions/index.md: medial-gap rule + Beowulf example.
• docs/guide/syntax.md: cross-reference updated.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.05%. Comparing base (6818161) to head (e0076bd).

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #244      +/-   ##
============================================
+ Coverage     92.04%   92.05%   +0.01%     
- Complexity     3559     3566       +7     
============================================
  Files           107      107              
  Lines         10078    10093      +15     
============================================
+ Hits           9276     9291      +15     
  Misses          802      802              

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