Implement z-index-aware hit testing, overflow clipping, keyboard default behaviors
(Enter/Space on links/buttons/checkboxes), horizontal and keyboard page scrolling,
container-level overflow:auto/scroll with scroll chaining, and scroll event dispatch.
Code review fixes: scroll events target actual scrolled container element, deduplicated
link navigation via navigate_link_href() helper, and reference-based scroll offset API.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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