refactor(grades,diagnostic): 完成成绩和学情诊断模块审计 P1+P2 改进项

P1-1: 抽取 stats-service.ts，将 8 个统计计算纯函数从 data-access 层分离
P1-5: 创建 WidgetBoundary 组件 + 补齐 teacher 路由 loading.tsx/error.tsx (14 文件)
P1-6: 同步架构图文档 004/005，新增 stats-service 与 widget-boundary 节点
P2-1: 补充 a11y ARIA 属性（5 图表 role=img + aria-label，2 表格 caption，3 列表 role=list，3 按钮 aria-label）
P2-3: 修复班级报告 studentId 字段语义错误（schema 改为可空 + 迁移 + 代码适配）
P2-4: 修复 grade_managed scope 返回空数据（改为子查询 classes 表按 gradeId 过滤）
P2-5: 新增 /parent/diagnostic/ 页面（多子女学情诊断聚合 + loading + error）
P2-6: 统一 SearchParams 工具（student/grades 和 management/grade/insights 改用 @/shared/lib/search-params）

src/modules/diagnostic/components/student-diagnostic-view.tsx

@@ -96,7 +96,7 @@ export function StudentDiagnosticView({ summary, reports, classAverageMastery }:
             {summary.strengths.length === 0 ? (
               <p className="text-sm text-muted-foreground">No strengths identified yet.</p>
             ) : (
-              <ul className="space-y-2">
+              <ul className="space-y-2" role="list" aria-label="优势知识点列表">
                 {summary.strengths.map((m) => (
                   <li key={m.knowledgePointId} className="flex items-center justify-between">
                     <span className="text-sm">{m.knowledgePointName}</span>
@@ -119,7 +119,7 @@ export function StudentDiagnosticView({ summary, reports, classAverageMastery }:
             {summary.weaknesses.length === 0 ? (
               <p className="text-sm text-muted-foreground">No weaknesses identified.</p>
             ) : (
-              <ul className="space-y-2">
+              <ul className="space-y-2" role="list" aria-label="薄弱知识点列表">
                 {summary.weaknesses.map((m) => (
                   <li key={m.knowledgePointId} className="flex items-center justify-between gap-2">
                     <div className="flex items-center gap-2 min-w-0">
@@ -162,7 +162,7 @@ export function StudentDiagnosticView({ summary, reports, classAverageMastery }:
             {latestReport.recommendations && latestReport.recommendations.length > 0 ? (
               <div>
                 <h4 className="mb-2 text-sm font-semibold">Recommendations</h4>
-                <ul className="space-y-1.5">
+                <ul className="space-y-1.5" role="list" aria-label="学习建议列表">
                   {latestReport.recommendations.map((rec, i) => (
                     <li key={i} className="text-sm text-muted-foreground">• {rec}</li>
                   ))}