fix: clarify single-page toc detection

[Bortlesboat]

Summary

• clarify the single-page TOC detector prompt so structured document content is not treated as a table of contents just because it has numbered sections
• call out that a true table of contents should reference content elsewhere in the document
• add a regression test that captures the prompt sent to the detector

Verification

• RED: python -m pytest tests/test_toc_detector.py -q failed before the prompt guidance was added
• GREEN: python -m pytest tests/test_toc_detector.py -q
• python -m compileall -q pageindex tests
• git diff --check -- pageindex/page_index.py tests/test_toc_detector.py

Closes #203