fix: keep the last right-nav section highlighted

[voidborne-d]

Summary

• treat TOC sections as active as soon as they enter the reading viewport instead of waiting for a very high intersection ratio
• prefer the first visible heading below the top edge, with a fallback that keeps the last visible heading active when all visible headings are already above the top
• add regression coverage for the short-final-section case reported in Right navigation last link not highlighting when clicked #3895

Testing

• Added packages/gitbook/src/components/hooks/useScrollActiveId.test.ts
• bun test packages/gitbook/src/components/hooks/useScrollActiveId.test.ts --preload ./packages/gitbook/tests/preload-bun.ts (could not run here because bun is not installed in this environment)
• Node smoke check for the new selection logic

[changeset-bot]

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Latest commit: 36f43b3

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