fix: correct section page ranges at page-leading headings

[KumamuKuma]

Summary

• close the previous section on the preceding page when a new page starts with a recognized section heading
• preserve overlapping page coverage when the heading appears after section content on the same page
• add focused regressions for page-leading and mid-page headings

Root cause

extract_raw_sections() advanced the current section's page_end before examining the first meaningful line on each page. When that line was a new section heading, the previous section was therefore recorded as covering the new section's page as well.

This caused section_ids_for_page() and primary_section_for_page() to expose the previous section on a page that begins with a new heading, which could bind captions on that page to the wrong section.

Validation

• before the fix, the page-leading regression failed with Introduction.page_end == 2 instead of 1
• python -m pytest -q tests/test_extract_source_text.py — 6 passed
• python -m ruff check --no-cache --select F,I skills/deeppapernote/scripts/extract_source_text.py tests/test_extract_source_text.py — passed
• python -m compileall -q skills/deeppapernote/scripts — passed
• python -m pytest -q — 827 passed, 1 skipped
• independent diff review found no blocking issues

Scope

This remains page-granular by design. It does not change caption attribution for captions after a mid-page heading, which would require line-position-aware parsing.