feat: support paragraph between borders (w:pBdr/w:between)

[gpardhivvarma]

Summary

Closes #2074

• Flow w:between borders through the full rendering pipeline so consecutive paragraphs sharing the same border definition display a horizontal separator line between them
• Add between to ParagraphBorders contract type and normalizeParagraphBorders side iteration
• Add computeBetweenBorderFlags to pre-compute which fragments need the between border (following the computeSdtBoundaries pattern)
• Render between border as CSS border-bottom on the decoration overlay, with support for header/footer sections, page splits, and patch invalidation

Test plan

• normalizeParagraphBorders includes between side (pm-adapter tests)
• hashParagraphBorders includes bw: segment (both hash util files)
• computeBetweenBorderFlags — matching borders, non-matching, page splits, same-block dedup, list items, mixed types, chains, empty/single fragments, missing lookup, asymmetric borders
• applyParagraphBorderStyles — activation, bottom border overwrite, partial specs, defaults
• getFragmentParagraphBorders — para, list-item, missing data, kind mismatch
• 55+ unit tests across 4 test files, all passing

[caio-pizzol]

@gpardhivvarma solid work — logic looks correct and well-tested with 55+ unit tests.

two things:

1. the incremental update path (renderer.ts:2207) where a fragment switches in/out of a between-border group isn't tested. there's a good example to follow in clip-path-cache-invalidation.test.ts — render, change the data, re-render, check the element got replaced.

2. we'll add a test document with between borders to the visual test suite on our end.

[gpardhivvarma]

@caio-pizzol please review this when you get the time.

[caio-pizzol]

Closing in favor of #2324, which builds on this work with an improved between-border group model and extracts the rendering logic into a dedicated feature module (features/paragraph-borders/).

Thanks for the initial implementation @gpardhivvarma!