IBookParser class abstract interface

Parses a book file's bytes into the shared document model.

Both methods are Future-returning but do no I/O — the work is CPU-bound from start to finish. Awaiting parse on a large book blocks the isolate it runs on, so callers with a user interface should run it through Isolate.run; everything inside the returned document is sendable as long as any caller-supplied TextSegmenter holds plain data only.

Note that segmentation is not moved into the isolate by moving the parse into it: it is lazy, and runs on whichever isolate first touches ParagraphBlock.sentences. Images, in contrast, are eager and resident.