Add ChapterKind for front/back matter chapters
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.
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