fix: a footnote reference may wrap, and labels normalize before lookup

[dereuromark]

Follows the upstream ruling in jgm/djot.js#146: the two sides of a footnote label are deliberately asymmetric.

• A reference may contain a line break. It is normalized before lookup, so a reference a text editor has wrapped still binds.
• A definition marker may not. It stays a single-line construct the block parser can find without scanning ahead.

PR 267 here took the strict reading on both sides. The definition half was right and stays; the reference half is reverted, and the piece that makes it work is added.

The real gap was normalization, not the newline

Footnote labels were never normalized in this parser at all. Measured against the djot.js reference implementation, version 0.3.2, before this change, with [^a b]: foo as the definition:

reference	djot.js	djot-php before
[^a b]	binds	does not bind
[^a<TAB>b]	binds	does not bind
[^ a ]	binds	does not bind
[^a<NEWLINE>b]	binds on main	does not bind

Only the last row is what the upstream issue is about; the first three were already divergent and nothing here covered them.

Link references did not have this problem, which is why it stayed invisible: BlockParser's [label]: url branch normalizes the definition, and the inline side normalizes the reference. The footnote path had neither.

Both are added now, so the two sides meet on the same key:

see[^two
   words].

[^two words]: note.

<p>see<a id="fnref1" href="#fn1" role="doc-noteref"><sup>1</sup></a>.</p>
<section role="doc-endnotes">
<hr>
<ol>
<li id="fn1">
<p>note<a href="#fnref1" role="doc-backlink">↩︎</a></p>
</li>
</ol>
</section>

And the definition marker still does not cross a line ending:

see[^two words].

[^two
words]: note.

No footnote is registered; the marker line stays a paragraph.

Character class

StringUtil::normalizeLabel() is djot.js's normalizeLabel character for character - trim() plus [ \t\r\n]+ collapsed to one space - rather than PHP's \s, which also covers form feed and vertical tab. That difference is observable rather than academic: given [a b]: url,

[t][a<FORM FEED>b]

djot.js leaves the reference unresolved, a \s-based normalizer binds it.

The two existing link-side call sites do use \s and still diverge on that one character. Pre-existing, and deliberately not folded into a footnote fix - happy to do it as a follow-up if you want the parity.

One shape changes appearance

A definition marker that IS wrapped stays a paragraph, and a paragraph is inline-parsed like any other - so its own [^a / b] is now a valid (and usually unresolved) reference rather than literal text. The definition still does not register, which is the part that matters, but the rendered paragraph shows a note marker where it previously showed the raw source.

Notes

• The AST carries the normalized label, matching djot.js, so a wrapped reference reports its undefined-footnote warning under the label it actually resolved to.
• FootnoteLabelIsSingleLineTest is replaced by FootnoteLabelNormalizationTest: same definition-side coverage, plus the three line endings on the reference side and the whitespace variants on both sides.
• No docs page states the old rule, so there is nothing to update there.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.41%. Comparing base (f90149f) to head (25ea9d0).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #269      +/-   ##
============================================
- Coverage     92.41%   92.41%   -0.01%     
- Complexity     3673     3674       +1     
============================================
  Files           109      109              
  Lines         10421    10424       +3     
============================================
+ Hits           9631     9633       +2     
- Misses          790      791       +1     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.