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/app/(dashboard)/parent/diagnostic/error.tsx

@@ -0,0 +1,27 @@
+"use client"
+
+import { AlertCircle } from "lucide-react"
+
+import { EmptyState } from "@/shared/components/ui/empty-state"
+
+export default function ParentDiagnosticError({
+  reset,
+}: {
+  error: Error & { digest?: string }
+  reset: () => void
+}) {
+  return (
+    <div className="flex h-full flex-col items-center justify-center space-y-4 p-8">
+      <EmptyState
+        icon={AlertCircle}
+        title="子女学情诊断加载失败"
+        description="抱歉，加载子女诊断数据时发生了意外错误。请稍后重试。"
+        action={{
+          label: "重试",
+          onClick: () => reset(),
+        }}
+        className="border-none shadow-none h-auto"
+      />
+    </div>
+  )
+}

src/app/(dashboard)/parent/diagnostic/loading.tsx

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+import { Card, CardContent, CardHeader, CardTitle } from "@/shared/components/ui/card"
+import { Skeleton } from "@/shared/components/ui/skeleton"
+
+export default function Loading() {
+  return (
+    <div className="space-y-8 p-6 md:p-8">
+      <div className="space-y-2">
+        <Skeleton className="h-8 w-48" />
+        <Skeleton className="h-4 w-64" />
+      </div>
+      <div className="space-y-8">
+        {Array.from({ length: 2 }).map((_, i) => (
+          <Card key={i}>
+            <CardHeader>
+              <CardTitle className="text-lg">
+                <Skeleton className="h-5 w-32" />
+              </CardTitle>
+            </CardHeader>
+            <CardContent className="space-y-4">
+              <Skeleton className="h-72 w-full" />
+              <div className="grid gap-4 md:grid-cols-2">
+                {Array.from({ length: 2 }).map((_, j) => (
+                  <Skeleton key={j} className="h-40 w-full" />
+                ))}
+              </div>
+            </CardContent>
+          </Card>
+        ))}
+      </div>
+    </div>
+  )
+}

src/app/(dashboard)/parent/diagnostic/page.tsx

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+import { Stethoscope } from "lucide-react"
+
+import { requirePermission } from "@/shared/lib/auth-guard"
+import { Permissions } from "@/shared/types/permissions"
+import { getStudentMasterySummary } from "@/modules/diagnostic/data-access"
+import { getDiagnosticReports } from "@/modules/diagnostic/data-access-reports"
+import { StudentDiagnosticView } from "@/modules/diagnostic/components/student-diagnostic-view"
+import {
+  ParentChildrenDataPage,
+  ParentNoChildrenPage,
+} from "@/modules/parent/components/parent-children-data-page"
+
+export const dynamic = "force-dynamic"
+
+export default async function ParentDiagnosticPage() {
+  const ctx = await requirePermission(Permissions.DIAGNOSTIC_READ)
+
+  if (ctx.dataScope.type !== "children" || ctx.dataScope.childrenIds.length === 0) {
+    return (
+      <ParentNoChildrenPage
+        title="Children Diagnostic"
+        description="View your children's knowledge point mastery and diagnostic reports."
+        icon={Stethoscope}
+        emptyTitle="No children linked"
+        emptyDescription="Your account is not linked to any student accounts yet. Please contact the school administrator."
+      />
+    )
+  }
+
+  // 使用 allSettled 容错：单个子女查询失败不影响其他子女展示
+  const results = await Promise.allSettled(
+    ctx.dataScope.childrenIds.map(async (id) => {
+      const [summary, reports] = await Promise.all([
+        getStudentMasterySummary(id),
+        getDiagnosticReports({ studentId: id }),
+      ])
+      return { summary, reports, studentId: id }
+    }),
+  )
+  const validItems = results
+    .filter(
+      (r): r is PromiseFulfilledResult<{
+        summary: Awaited<ReturnType<typeof getStudentMasterySummary>>
+        reports: Awaited<ReturnType<typeof getDiagnosticReports>>
+        studentId: string
+      }> => r.status === "fulfilled",
+    )
+    .map((r) => r.value)
+
+  return (
+    <ParentChildrenDataPage
+      title="Children Diagnostic"
+      description="View knowledge point mastery and diagnostic reports for all your children."
+      icon={Stethoscope}
+      noRecordsTitle="No diagnostic data"
+      noRecordsDescription="Your children don't have any diagnostic data yet."
+      items={validItems}
+      renderItem={({ summary, reports }) => (
+        <>
+          <div className="border-b pb-2">
+            <h3 className="text-lg font-semibold">
+              {summary?.studentName ?? "Unknown student"}
+            </h3>
+          </div>
+          <StudentDiagnosticView summary={summary} reports={reports} />
+        </>
+      )}
+    />
+  )
+}