fix(parser): stop section extraction at any heading or thematic break (#2555 task 2)

[jung-thomas]

Follow-up to #2556. Addresses the second, rarer parse-time issue in #2555.

Problem

extractSection and extractBulletList (scripts/parsers/frontmatter.ts) bounded a section only at the next ## /### heading:

(?=\n## |\n### |$)

So a ## Prerequisites / ## You will learn block over-captured everything down to EOF whenever the content that followed it had no h2/h3 — e.g. an #### sub-heading, a # H1, or a bare ----separated intro paragraph. Rare in practice because authors almost always put ## Steps or a ### Step 1 right after, which correctly bounds the section.

Distinct from #2556, which was a render-time heading duplication; this is a parse-time boundary in the frontmatter extractor.

Fix

Widen the boundary to stop at any ATX heading (#..######) or a thematic break (---/***/___ alone on a line):

(?=\n#{1,6} |\n(?:---|\*\*\*|___)\s*(?:\n|$)|$)

Applied to both extractors via a shared SECTION_END constant.

Safety invariant: every new alternative is an earlier-or-equal stop point, so the change can only ever shrink an over-capturing section, never grow one. A section correctly bounded by ## Steps today still stops there — no correctly-rendering tutorial can regress.

Testing

npx vitest run — 19/19 pass:

• New extract-section-boundary.test.ts (5 cases): stops at ####, at bare ---, at # H1, a control proving ## Steps-bounded prereqs still capture fully, and the same boundary honoured by extractBulletList.
• Existing comment-aware-extraction.test.ts + frontmatter-utils.test.ts stay green.

No local corpus diff was possible (no .tutorial-cache/ present); the "can only shrink" invariant plus CI + the post-DEV e2e job are the backstop.