refactor(epub): rebuild iOS page-curl mode on one web view per chapter

[everpcpc]

What

Rebuilds the iOS page-curl transition on the same single-web-view-per-chapter architecture the cover mode moved to in #1236, instead of one WKWebView per page.

• One live EpubPageViewController per chapter plus preloaded boundary neighbors (at most 3 web views), replacing the 5-entry per-page controller cache, the pendingControllers/reservedControllers bookkeeping with its 0.3s cleanup timer, and the backside-image LRU.
• UIPageViewController slots become dumb shells (CurlPageShellViewController): a chapter-internal flip covers the front shell with a snapshot of the current page and migrates the live web view into a pre-sized target shell scrolled to the target page. The curling leaf is a static image, the mirrored back uses the existing PageCurlBacksideViewController, and the reveal plus the committed page are the same live view.
• Cross-chapter flips keep the live leaf and reveal the preloaded neighbor chapter; roles rotate on commit.
• Interactive turns commit or restore in didFinishAnimating; programmatic turns commit in their own setViewControllers completion. The internal pan recognizers carry a target that cancels dataSource prefetches abandoned before a curl starts, so a dropped gesture can never wedge the turn state.
• Tap zones, book-end refusal with onEndReached, RTL spine/semantic mapping, backside style, overlay labels, and progress recording behave as before.

Why

The per-page model held up to 5 whole-chapter DOMs in separate web content processes, and every in-chapter page turn re-injected CSS and re-ran pagination for a chapter already laid out next door. One web view per chapter removes both costs and deletes the compensating bookkeeping around UIPageViewController's opaque retention.

Validation

• make build passes on all platforms.
• iOS simulator with an English EPUB: tap turns both directions; interactive curls both directions with mid-gesture captures (static leaf, mirrored leaf back, live revealed page already scrolled to the target); short-drag cancels returning to the exact origin page; cross-chapter curls both directions with the neighbor pre-scrolled to its first/last page; book-start boundary refuses to curl; single-page chapter crossing; rotation reflow.
• Not covered: RTL books (spine/semantic/direction mapping carried over unchanged) and the book-end onEndReached path (same unchanged boundary check as book start).

Part of #1234