feat(attendance,elective): 实现所有 P2 长期改进项

P2 修复（来自审计报告）：
- 2.4.4: Server Action 错误消息 i18n 化（attendance/elective 全部 Action）
- 2.5.3: 抽取 AttendancePageLayout 组件复用（admin/teacher 页面）
- 2.5.4: 抽取 ElectivePageLayout 组件复用（admin/teacher 列表页）
- 2.6.3: 考勤月历键盘导航（tabIndex + 方向键 + Home/End + role=grid）
- 2.8.2: getStudentAttendanceSummary 分页优化（SQL 聚合统计 + LIMIT 分页）
- 2.8.3: resolveCourseDisplayNames 缓存优化（React cache 去重）
- 2.1.4: elective data-access 跨模块依赖接口抽象（resolvers.ts 可注入）

P2 建议项：
- 选课时间冲突检测（parseSchedule + isScheduleConflict 纯函数 + checkScheduleConflict）
- 学分上限校验（MAX_CREDIT_PER_TERM + checkCreditLimit）
- 考勤/选课数据导出 Excel（export.ts + API 路由扩展）

新增文件：
- src/modules/attendance/components/attendance-page-layout.tsx
- src/modules/elective/components/elective-page-layout.tsx
- src/modules/elective/resolvers.ts
- src/modules/attendance/export.ts
- src/modules/elective/export.ts

校验：
- npm run lint 通过（exit 0）
- npx tsc --noEmit attendance/elective/parent 相关零错误

src/modules/textbooks/graph-layout.test.ts

@@ -60,5 +60,69 @@ describe("textbooks/graph-layout", () => {
         expect(node.position.y).toBeGreaterThanOrEqual(0)
       }
     })
+
+    it("should not create edge for non-existent parentId", () => {
+      const layout = computeGraphLayout([makeKp("1", "nonexistent")])
+      expect(layout.nodes).toHaveLength(1)
+      expect(layout.edges).toHaveLength(0)
+    })
+
+    it("should not create edge for non-existent prerequisiteIds", () => {
+      const layout = computeGraphLayout([makeKp("1", null, ["nonexistent"])])
+      expect(layout.nodes).toHaveLength(1)
+      expect(layout.edges).toHaveLength(0)
+    })
+
+    it("should generate both parent and prerequisite edges for one node", () => {
+      const layout = computeGraphLayout([
+        makeKp("1"),
+        makeKp("2"),
+        makeKp("3", "1", ["2"]),
+      ])
+      const parentEdge = layout.edges.find((e) => e.id === "parent-1-3")
+      const prereqEdge = layout.edges.find((e) => e.id === "prereq-2-3")
+      expect(parentEdge).toBeDefined()
+      expect(prereqEdge).toBeDefined()
+      expect(layout.edges).toHaveLength(2)
+    })
+
+    it("should return positive width/height for non-empty graph", () => {
+      const layout = computeGraphLayout([
+        makeKp("1"),
+        makeKp("2", "1"),
+        makeKp("3", "1"),
+      ])
+      expect(layout.width).toBeGreaterThan(0)
+      expect(layout.height).toBeGreaterThan(0)
+    })
+
+    it("should handle deeply nested parent chain (boundary)", () => {
+      const depth = 20
+      const kps: KpWithRelations[] = [makeKp("0")]
+      for (let i = 1; i < depth; i++) {
+        kps.push(makeKp(String(i), String(i - 1)))
+      }
+      const layout = computeGraphLayout(kps)
+      expect(layout.nodes).toHaveLength(depth)
+      // 应生成 depth-1 条 parent 边
+      const parentEdges = layout.edges.filter((e) => e.id.startsWith("parent-"))
+      expect(parentEdges).toHaveLength(depth - 1)
+    })
+
+    it("should handle many nodes (boundary, 50 nodes)", () => {
+      const count = 50
+      const kps: KpWithRelations[] = []
+      for (let i = 0; i < count; i++) {
+        kps.push(makeKp(`n${i}`))
+      }
+      const layout = computeGraphLayout(kps)
+      expect(layout.nodes).toHaveLength(count)
+      expect(layout.edges).toHaveLength(0)
+      // 所有节点都应有有效位置
+      for (const node of layout.nodes) {
+        expect(Number.isFinite(node.position.x)).toBe(true)
+        expect(Number.isFinite(node.position.y)).toBe(true)
+      }
+    })
   })
 })