fix: keep lazy setext underlines in list items as text

[vhinayindia]

Marked version: 18.1.0

Markdown flavor: CommonMark

Description

CommonMark 0.31.2 §4.3 says a setext underline cannot be a lazy continuation line in a list item. For example:

- foo
===

Marked currently treats === as an underline and emits a heading. After the list tokenizer removes the item's indentation, the lazy line can incorrectly match the setext rule.

Keep an under-indented setext-looking continuation as paragraph text in this case, while preserving correctly indented setext headings. The regression fixtures cover both underline levels, zero and partial indentation, prior continuation text, and nested lists. PR #4119 addresses list tab-stop expansion; this change covers the separate setext parsing behavior.

Contributor

• Regression tests exist to ensure functionality and minimize regression.
• If submitting new feature, it has been documented in the appropriate places.

Verification

• npm test passed, including docs build, spec suite, 193 unit tests, UMD/CJS, type/export checks, and ESLint.
• A 125-case differential matrix against CommonMark.js matched with zero mismatches.

Committer

• CI is green (no forced merge required).
• Squash and Merge PR following conventional commit guidelines.

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[UziTech]

Nice catch! 💯