fix: strip whitespace when normalizing link reference labels

[NgoQuocViet2001]

Problem

CommonMark normalizes a link label in three steps — case-fold, strip leading and trailing whitespace, collapse internal runs. normalizeLabel does only the first:

export function normalizeLabel(label: string) {
  return label.toLowerCase().toUpperCase().toLowerCase();
}

The collapse is bolted onto one of its four call sites (Tokenizer.ts:531, the def rule); reflink and the two Lexer lookups don't have it, and nothing strips anywhere. A definition and a reference that differ only in surrounding whitespace therefore normalize to different keys and never match.

Compared against the commonmark 0.31.2 reference implementation already in devDependencies, normalizing away marked's <img> vs <img /> difference:

DIFF  trailing space in the reference       [foo ] + [foo]: /url
        marked     : <p>[foo ]</p>
        commonmark : <p><a href="/url">foo </a></p>
DIFF  leading space in the reference        [ foo] + [foo]: /url
DIFF  trailing space in the definition      [foo]  + [foo ]: /url
DIFF  emphasis around a spaced reference    *[foo ]* + [foo]: /url
        marked     : <p><em>[foo ]</em></p>
        commonmark : <p><em><a href="/url">foo </a></em></p>

4 of 9 probe inputs diverge. The collapse cases (internal double space, tab, newline) all pass — the missing step is the strip.

The failure is silent: a valid reference link renders as literal bracket text, and in the masking case the surrounding emphasis is disturbed too.

Fix

Do the strip inside normalizeLabel, so all four call sites agree by construction.

return label.trim().toLowerCase().toUpperCase().toLowerCase();

Test plan

• Added test/specs/new/link_reference_label_whitespace.{md,html}. The expected HTML is the commonmark reference implementation's own output for that input, not hand-written.
• Ran: node --test test/run-spec-tests.js → 1815 passing (1813 before, plus the new pair).
• Ran: node --test test/unit/*.test.js → 191 passing.
• Checked: reverting only helpers.ts fails the new spec. Worth noting that the existing 1813 pass identically with and without this change — the CommonMark spec files in the repo don't cover surrounding whitespace in a label, which is why this survived.
• Ran: eslint src/helpers.ts → clean.

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Project	Deployment	Actions	Updated
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[UziTech]

Nice catch! 💯