Core utilities

A @prose-reader/core helper: observeBookBoundaryReached() tells when the user reaches the start or end of the book, holding the end back while items load.

`observeBookBoundaryReached(reader)`

Product-level "user reached the start/end of the book" signal with last-item readiness so "end" events are withheld while the spine is still growing through lazy loads. "start" passes through immediately.

observeBookBoundaryReached(reader)