Touch drags flip pages: the page follows the finger live, the axis locks
on first movement so vertical pans never flip, and edge drags get
rubber-band resistance. Swipes starting on interactive controls are
ignored. Swipe intent lives in bookSwipeDirection with unit tests.
Fullscreen reader alongside Immersive view: paginated two-page spread
with no vertical scroll, chevron and arrow-key page turns, and a page
indicator. Chapters always render in reading order (Chapter 1 first).
Layout knobs (spine gap, single-column breakpoint, book max width) live
in lib/story/bookLayout with unit-tested paging math.
