Validate mixed-section PDF geometry (#440)

[JSv4]

Closes #440.

Stacked on #502 (agent/issue-439-pdf-export). This PR remains draft until its dependencies land.

Summary

• preserve section ownership and physical dimensions across continuous-section sharing and spill pages
• make displayed page numbering and first/even/odd story selection follow section restart semantics
• retain pre-boundary footnotes on their owning page and precisely exclude only zero-height explicit-break carriers from PageMap
• validate PDF MediaBox and CropBox origins and dimensions within tolerance
• add a generated six-page Letter/A4, portrait/landscape, explicit/next/continuous/multicolumn export fixture
• prove browser viewer zoom does not affect printed PDF geometry

Validation

• focused Playwright pagination suites: 29 passed
• npm-export: 7 passed
• TypeScript builds and package-boundary checks passed
• full Playwright matrix: 528 passed, 9 skipped, 5 unrelated existing tab-visual snapshot mismatches

Artifacts

The Actions playwright-report artifact is uploaded with if: always(). Download it and open npm-export/test-artifacts/view-artifacts.html to inspect the generated DOCX, standalone HTML, normal/scaled PDFs, screenshot, PageMap, render report, and PDF inspection JSON.