feat(exams): add section/group structure nodes with auto stats
Distinguish structural levels from questions:
- sectionBlock (new): top-level volume like "第Ⅰ卷 选择题(共24分)"
- groupBlock (enhanced): section like "一、选择题" with instruction field
- questionBlock (composite): reserved for reading comprehension
Key changes:
- New section-block.tsx extension with level attr (1=卷, 2=部分) and
auto-computed question count + total score in NodeView
- group-block.tsx: add instruction field ("每小题3分"), auto stats display
- editor-to-structure.ts: recursive buildStructureNode supports arbitrary
nesting (section > group > question), computeStats accumulates scores
- exam-rich-form.tsx ExamPreview: render section/group/question with
distinct styles and stats badges
- selection-toolbar.tsx: add "分卷" button (Layers icon)
- exam-rich-editor.tsx: register SectionBlock, expose insertSection/
wrapInSection via ref
- actions.ts: AI prompt now outputs volumes[] + groups[] structure with
instruction; buildTiptapDocFromAiResponse generates nested sectionBlock
- i18n: add markSection keys (zh-CN/en)
Structural nodes are NOT questions: their question count and total score
are automatically computed from child questions, not manually set.
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