Chunk documents properly

My documents: <format>, roughly <n> docs averaging <length>.

Write the chunking step:
- Split on structure — headings, sections, paragraphs — not a fixed
  character count that cuts sentences in half.
- Target <n> tokens with sensible overlap, so a fact spanning a
  boundary isn't lost.
- Keep with each chunk: source, heading path, position in document.
- Handle documents shorter than one chunk, and single sections longer
  than a chunk.

Then give me a way to print 20 random chunks so I can read them. I want
to check each would make sense to someone seeing it in isolation.