feat(diagnostic): add export, stats service, and confidence utils

- Add export module for diagnostic report data export

- Add stats-service for diagnostic analytics aggregation

- Add confidence-utils for diagnostic confidence score calculations

src/modules/diagnostic/components/confidence-utils.ts

@@ -0,0 +1,31 @@
+/**
+ * v4-P3-7: 诊断报告数据置信度工具。
+ *
+ * 置信度等级用于指示报告基于的数据量是否充足，帮助教师判断报告可信度。
+ * 提取到独立文件供 report-list 和 student-diagnostic-view 共享，避免重复定义。
+ */
+
+import type { DiagnosticReportWithDetails } from "../types"
+
+export type ConfidenceLevel = "high" | "medium" | "low" | "insufficient"
+
+/**
+ * 根据报告数据计算置信度。
+ * 简化方案：overallScore === null 表示无数据（insufficient），
+ * 否则视为高置信度（high）。
+ * 后续可扩展为基于 totalQuestions 等数据量字段的多级判断。
+ */
+export function getConfidenceLevel(report: DiagnosticReportWithDetails): ConfidenceLevel {
+  if (report.overallScore === null) return "insufficient"
+  return "high"
+}
+
+export const confidenceBadgeVariant: Record<
+  ConfidenceLevel,
+  "default" | "secondary" | "destructive" | "outline"
+> = {
+  high: "default",
+  medium: "secondary",
+  low: "destructive",
+  insufficient: "outline",
+}