Tapping a chapter in the artifact detail now opens a chapter detail screen.
- New ChapterDetailRoute + ChapterDetailScreen/ViewModel/UIState. The screen
shows the chapter's index and word/character counts, its original markdown
text (rendered as plain text — no markdown renderer available), and the
chapter's paragraph chunks (index, text, counts) with an empty state.
- DAO queries getChapterById and getChunksByChapterId (validated by Room),
exposed via MantraRepository.getChapter / getChunksForChapter (impl + NO_OP).
- ArtifactDetailScreen chapter cards are now clickable, navigating to
ChapterDetailRoute; MantraNavHost registers the route.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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