Fixes #3719. Picks up where #3909 left off, addressing @joshgoebel's review comments there.

*** on its own line is a thematic break, but BOLD was tried before HORIZONTAL_RULE in the top-level contains, so it matched ** and opened a strong span that swallowed the rest of the document.

Simply reordering was what stalled the previous attempt, because the old matcher (^[-\*]{3,} … $) was loose enough to also eat the opening *** of ***bold italic***. So this does both: anchors the rule to a full line, then moves it ahead of BOLD/ITALIC.

const HORIZONTAL_RULE = { match: /^ {0,3}([-*_])[ \t]*(?:\1[ \t]*){2,}$/ };

Per spec 0.31.2: up to three leading spaces, then the same marker three or more times, separated and followed by spaces or tabs, and nothing else on the line.

On the earlier review comments:

• _ style rule — covered; the marker is a capture group with a backreference, so ___ works and mixed runs like -*- correctly don't.
• \s*$ — deliberately not used: \s matches \n, which would let a break swallow following blank lines. [ \t]* matches the spec's wording.
• escaping * inside [] — not needed, dropped.

Behaviour is unchanged for ***bold italic***, **bold**, *italic*, setext headings (Heading / ---), and list bullets — all covered by the existing bold_italics and list tests, which still pass.

Out of scope, flagging it so it isn't a surprise: - - - still highlights as a bullet because LIST precedes HORIZONTAL_RULE. The spec says a thematic break wins there, but that's a separate precedence question and I'd rather not fold it into this fix. Happy to follow up.

npx mocha test → 1611 passing, 3 pending (was 1610 before the new test). Lint on src/languages/markdown.js is unchanged from main (one pre-existing spaced-comment error on the ENTITY comment, untouched here).

Checklist

• Added markup tests (test/markup/markdown/horizontal_rule.{txt,expect.txt})
• Updated CHANGES.md