Add opt-in source-line tracking for editor scroll-sync

[dereuromark]

What

Adds a sourceLines option (default off) that stamps each top-level block element with a data-source-line attribute holding the 1-based source line where the block started:

$html = (new DjotConverter(sourceLines: true))->convert("# Heading\n\nA paragraph.\n");

<section id="Heading">
  <h1 data-source-line="1">Heading</h1>
  <p data-source-line="3">A paragraph.</p>
</section>

Why

Editor live-preview scroll synchronization: read the line of the block at the top of the source pane, find the element whose data-source-line matches in the rendered pane, scroll it into view. The 1-based value matches editor gutters (Monaco / CodeMirror).

Details

• BlockParser gains a trackSourceLines constructor flag; the main block loop stamps the document's direct children via a small helper. The attribute is set on the node, so it renders through the normal attribute path (after author attributes, e.g. <p class="note" data-source-line="2">).
• DjotConverter exposes it as a sourceLines constructor option, threaded to the default parser (ignored when a pre-configured $parser is passed — set new BlockParser(trackSourceLines: true) instead).
• Off by default, so normal output is unchanged. Raw HTML blocks and comments are not stamped.
• Documented in the API reference (new "Source-line tracking" section) and the README.

Full suite green (2703 tests), phpstan and phpcs clean.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.37%. Comparing base (f70cf58) to head (71e2c97).

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #262      +/-   ##
============================================
+ Coverage     92.36%   92.37%   +0.01%     
- Complexity     3637     3643       +6     
============================================
  Files           109      109              
  Lines         10292    10307      +15     
============================================
+ Hits           9506     9521      +15     
  Misses          786      786              

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