prepareChapter

Lays out one chapter. This is where a book's time goes, so it is also the only thing a reader has to wait for: opening at chapter 20 prepares chapter 20, not chapters 0 to 20.

Pages laid out this way count as never used: a loader that walks the whole book behind the reader must not push the chapter being read out of the budget, so these are the first to drop.

Lays chapter out a step at a time, each step the parse of the chapter, its box tree, a child of a block in normal flow, or its pages, and calls checkpoint between two (#389). A viewer on a thread that must draw frames suspends in checkpoint once a frame's share of time is used, and the layout goes on after the frame. No lock is held across a checkpoint, so two callers for one chapter may each lay it out; the first to finish publishes its pages, as prepareChapter would.