Eager-load every page of the current chapter rather than a bounded
rolling window, so pages are fetched up front (the browser paces them
over its per-origin connection limit, top-to-bottom = reading order)
instead of filling in as they're scrolled onto. This drops the
scrollLeadIdx/eagerThrough machinery in favour of a plain
`loading="eager"`.
The first few pages of the *next* chapter are already warmed when the
reader nears the end of the current one (the next-chapter preloader), so
flipping over stays instant.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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