Rebased onto the atomic save work: book chapter replacement, canonicalize and
the writability probe, and every command write now check the roots before
they reach crate::atomic. IDML import no longer reads linked images itself
(the engine passes its checked reader), load_icc_file is gone (profiles
arrive as bytes), the script loader goes through the host reader, and
FontDb::load_document_fonts asks a caller-supplied check about the folder and
each font file, so a link that leads out of the read root is skipped.
Prepare and stage every chapter before a book update writes files. Preserve imported IDML sources, reject open native chapters and report completed paths after a later write failure.
Follow-up to #485.
PR #615.
aiongg
skylartaylor
Isaac Levine