Chapters can now be marked FrontMatter/Body/BackMatter. Number stays
the manuscript sort key for every chapter; the author-facing display
number is now computed per-request as the chapter's ordinal among
Body chapters only, so a foreword or afterword no longer shifts the
numbering of the rest of the book. Surfaced through the API, agent
toolset, and import toolset.
James Wampler
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