Section wise markdown export v2

[gurkerl83]

Context:

• This PR is based on next.
• Prior work exists in Convert paragraph content to markdown #506 (not merged into next), and is referenced here as the previous approach.

Top-level detail:

• Previous approach (PR Convert paragraph content to markdown #506) generated __hike.markdown from AST serialization, so upstream remark AST mutations were reflected but source fidelity was not guaranteed.
• This caused round-trip drift (formatting/line endings/GFM layout normalization).
• This change generates __hike.markdown from original source offsets, prioritizing source-faithful output and side effect free plugin results.

Additional changes:

• Thread original source through the remark transform path into section serialization.
• Compute markdown only from section content paragraphs using node offsets.
• Preserve flow-level <br /> spacing semantics:
  • leading breaks before the first paragraph,
  • one baseline separator newline plus extra newlines for intermediate <br />,
  • trailing breaks after the last paragraph.
• Keep markdown export opt-in via markdownEnabled (attribute behavior unchanged).
• Remove the extra markdown serialization dependency introduced in the previous approach.
• Add focused tests for:
  • source-preserving markdown capture,
  • behavior when markdownEnabled is not set,
  • <br /> spacing behavior.

[vercel]

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