feat: heading <section> wrapping (P1.2)

[dereuromark]

Impl side of carve PR #18 (grammar PART 9 §13). Adopts djot's <section> wrapper around headings.

Behavior

renderHtml now does a stateful section-wrapping pass over top-level blocks. Every heading opens a <section id="{slug}"> holding the heading and the content up to the next same-or-shallower heading. The id moves from the <h*> to the <section>; sections nest by heading level.

# Intro

Text.

## Background

More.

# Next

Body.

→

<section id="intro">
  <h1>Intro</h1>
  <p>Text.</p>
  <section id="background">
    <h2>Background</h2>
    <p>More.</p>
  </section>
</section>
<section id="next">
  <h1>Next</h1>
  <p>Body.</p>
</section>

Rules

• Adjacent same-level headings → sibling sections.
• Skipped levels nest by number (# H1 / ### H3 → h3-section two levels deep, no synthesized h2).
• Explicit {#id} on a heading goes to <section>; other heading attrs (classes, key-values) stay on <h*>.
• No-heading / empty docs emit zero <section>s.
• Nested headings inside containers (blockquote, list item) are not top-level, carry no id, and stay bare <h*> — resolveHeadingIds only assigns ids to top-level headings.
• Fragment URLs (#slug), </#id> cross-references, and [Heading][] implicit references all keep working — only the id emission site moved. Browsers resolve #slug to whichever element carries the id.

Mirrors djot-php's renderDocumentWithSections shape.

Tests

• New test/section-wrapper.test.ts (11 cases): single/nested/sibling/skip-level wrapping, explicit-id placement, no-heading + empty docs, end-of-doc closing, crossref fragment resolution, nested-heading-not-wrapped.
• Updated heading-ids + implicit-heading-refs assertions to the section shape.
• 6 heading corpus fixtures (02-headings*, 17-attributes, 19-heading-ids) temporarily added to KNOWN_GAPS: the impl is now ahead of the vendored spec corpus, which still encodes the bare-<h*> shape. They are re-added once the carve repo re-vendors carve-lib and rewrites those fixtures, then bumps the spec submodule here. Same coordination pattern as the ASCII-slug change (ASCII-safe heading id slugs (baked Unicode -> ASCII translit) #13 → chore: bump spec submodule (ASCII heading-ids corpus + new examples) #15).

222 tests pass + 6 todo; typecheck clean.

Follow-up sequence

1. (this PR) carve-js renderer + tests
2. carve repo: re-vendor carve-lib + rewrite 02-headings* / 17-attributes / 19-heading-ids fixtures to section shape
3. carve-js: bump spec submodule + un-skip the 6 KNOWN_GAPS entries
4. carve-php: implement the same section wrapping