Suppress paragraph space-before at page tops

[JSv4]

Summary

• model Word paragraphs explicitly in measured pagination blocks, including outline-level paragraphs rendered as h1–h6
• centralize page-top margin resolution so fit calculations and painted clones make the same section-aware decision
• suppress paragraph space-before only for the first paragraph on later pages of the same section
• preserve document/section-start spacing, ordinary inter-paragraph collapsing, and non-paragraph block margins
• add a generated four-page DOCX regression for natural overflow, w:pageBreakBefore, section starts, and same-page spacing, plus direct hard-break/heading/non-paragraph probes
• record the tracked legal-contract improvement and update its live disposition

Why this belongs in the paginator

The converter must preserve the authored w:spacing/@w:before; whether Word paints it depends on where pagination places the paragraph. The paginator now records paragraph semantics during measurement and applies one page-placement rule to keep-with-next chain sizing, ordinary body budgeting, fresh-page fallback, and the cloned block that is rendered.

The rule is section-aware: page 1 of a document or section retains its authored space-before, while the first Word paragraph on a later page suppresses it. Tables and other block margins are not treated as paragraph spacing.

Verification

• npm run typecheck
• 42/42 pagination Playwright tests
• 52/52 generated regression, ratchet-contract, and Word-reference-contract checks
• npm run test:package-boundary (163 runtime/license files)
• filtered legal-contract visual-parity run: 3/3 pages, no conversion errors
  • page 2/3 first ink: 115 px before → 99 px after (Word 100; LibreOffice 99)
  • mean SSIM: 0.69467 → 0.72874
  • mean tolerant ink F1: 0.57203 → 0.70363
  • page 2 SSIM / ink F1: 0.61400 / 0.52821 → 0.66282 / 0.68928
  • page 3 SSIM / ink F1: 0.74653 / 0.52440 → 0.79992 / 0.75813

The visual rerun used the contracted LibreOffice 25.8.7.3 and Chromium 143. Poppler was 25.03 rather than the committed ratchet's 24.02, so the full-corpus ratchet was intentionally not rewritten; the recorded comparison is before/after in the same environment.

Fixes #428