example: add interactive Endor student grades center

docs/student-grades-demo.md

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+# 学生成绩管理中心测试页
+
+源码：[student-grades.endor](../examples/student-grades.endor)。
+
+在 Tessera 按 F12 打开 Console，点击 `>` 输入位置，粘贴完整源码后按 Enter。
+源码创建并渲染一个独立的「学生成绩管理中心」Page，不替换其他页面。
+
+该示例使用 6 位虚构学生的内存对象，展示语文、数学、英语和复核状态。
+它验证 Endor 类、原生 Column 子类的测量、自定义绘制、字段绑定、事件和
+同一对象在筛选前后的共享状态。关闭应用后数据不会保存。
+
+## 已完成的原生检查
+
+- 通过原生 Console 粘贴完整源码并按 Enter，成功出现成绩页标签和 6 行数据。
+- 「查看复核记录」切换为林沐的一行复核记录。
+- 「录入复核成绩」执行示例中预设的复核结果：数学 58 → 68、状态改为已复核、待复核 1 → 0。
+- 「全部学生」恢复 6 行，林沐的 68 分和已复核状态保留。
+- 「恢复示例数据」恢复 58 分、待复核状态和计数 1。
+
+这是交互测试示例，复核按钮应用预设结果，不是通用成绩录入表单；没有
+文件存储、通用校验、导入导出、真实学校数据或完整 CRUD 的实现声明。
+
+## 录制结果
+
+尝试了 macOS screencapture 的窗口录像与限定区域录像，均返回 capture error。
+随后尝试 QuickTime「新建录屏」，没有成功出现可操作的录制界面；直接打开
+系统录屏工具也超时。因此没有成功生成连续录屏，不把截图当作录屏交付。
+
+原生操作关键截图保存在本次会话的 /private/tmp/endor-grades-demo/：
+01-endor-input.png、02-rendered.png、03-filtered.png、04-reviewed.png、
+05-all-reviewed.png、06-reset.png。
+
+本次仅新增示例和此说明，未修改渲染器。以实际 Console 执行和原生点击
+作为验证；未运行 cargo test、自动化 smoke 脚本，也未因纯示例新增而重复构建。

examples/student-grades.endor

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+// Student grades demo. Fictional records; in-memory only.
+// Paste the entire file into Tessera Console, then press Enter.
+class GradeCell: Column {
+    width: Number
+    function measure(self: GradeCell, mount: Mount, constraints: Constraints): MeasureResult {
+        size = Column.measure(self, mount, constraints)
+        size.width = self.width
+        return size
+    }
+}
+class GradePanel: Column {
+    function paint(self: GradePanel, mount: Mount, geometry: Bounds): DisplayList {
+        fill = PaintCommand()
+        fill.kind = "roundedRect"
+        fill.x = geometry.x
+        fill.y = geometry.y
+        fill.width = geometry.width
+        fill.height = geometry.height
+        fill.radius = 12
+        fill.color = "#FFFFFF"
+        return DisplayList(ObjectList(fill))
+    }
+}
+class Student {
+    name: String
+    number: String
+    chinese: Number
+    math: Number
+    english: Number
+    status: String
+}
+function gradeCell(content: Block, width: Number): Block {
+    return GradeCell(0, 0, content, width)
+}
+function gradeRow(student: Student): Block {
+    return Column(0, 12,
+        Row(0, 0,
+            gradeCell(TextBlock(student.name), 150),
+            gradeCell(TextBlock(student.number, 1, "#7A8390"), 120),
+            gradeCell(ui.textOf(student, "chinese"), 100),
+            gradeCell(ui.textOf(student, "math"), 100),
+            gradeCell(ui.textOf(student, "english"), 100),
+            gradeCell(ui.textOf(student, "status", 1, "#526C83"), 160)),
+        RectBlock(730, 1, 0, "#EDF0F3"))
+}
+class GradeCenter {
+    table: Column
+    all: Column
+    attention: Column
+    student: Student
+    pending: Number
+    message: String
+    function showAll(self: GradeCenter, event: Event): Unit {
+        self.table.replace(0, self.all)
+        self.message = "正在查看全部 6 位学生"
+    }
+    function showAttention(self: GradeCenter, event: Event): Unit {
+        self.table.replace(0, self.attention)
+        self.message = "林沐的数学成绩有一条复核记录"
+    }
+    function review(self: GradeCenter, event: Event): Unit {
+        self.student.math = 68
+        self.student.status = "已复核"
+        self.pending = 0
+        self.message = "已录入复核结果：林沐 · 数学 58 → 68"
+    }
+    function reset(self: GradeCenter, event: Event): Unit {
+        self.student.math = 58
+        self.student.status = "待复核"
+        self.pending = 1
+        self.table.replace(0, self.all)
+        self.message = "已恢复示例数据"
+    }
+}
+studentLin = Student("林沐", "2026001", 86, 58, 91, "待复核")
+studentChen = Student("陈知夏", "2026002", 94, 96, 93, "已确认")
+studentXu = Student("许星河", "2026003", 82, 88, 85, "已确认")
+studentZhou = Student("周予安", "2026004", 91, 89, 95, "已确认")
+studentSu = Student("苏映雪", "2026005", 88, 92, 90, "已确认")
+studentLu = Student("陆景行", "2026006", 79, 84, 87, "已确认")
+allGradeRows = Column(0, 14, gradeRow(studentLin), gradeRow(studentChen), gradeRow(studentXu), gradeRow(studentZhou), gradeRow(studentSu), gradeRow(studentLu))
+attentionGradeRows = Column(0, 14, gradeRow(studentLin))
+gradeTable = Column(0, 0, allGradeRows)
+gradeCenter = GradeCenter(gradeTable, allGradeRows, attentionGradeRows, studentLin, 1, "正在查看全部 6 位学生")
+showAllGrades = Button("全部学生")
+showAttentionGrades = Button("查看复核记录")
+reviewGrade = Button("录入复核成绩")
+resetGrades = Button("恢复示例数据")
+showAllGrades.on("click", gradeCenter.showAll)
+showAttentionGrades.on("click", gradeCenter.showAttention)
+reviewGrade.on("click", gradeCenter.review)
+resetGrades.on("click", gradeCenter.reset)
+gradeHeader = Row(0, 0,
+    gradeCell(TextBlock("学生", 1, "#7A8390"), 150),
+    gradeCell(TextBlock("学号", 1, "#7A8390"), 120),
+    gradeCell(TextBlock("语文", 1, "#7A8390"), 100),
+    gradeCell(TextBlock("数学", 1, "#7A8390"), 100),
+    gradeCell(TextBlock("英语", 1, "#7A8390"), 100),
+    gradeCell(TextBlock("状态", 1, "#7A8390"), 160))
+gradeSummary = Row(0, 40,
+    Column(0, 6, TextBlock("学生人数", 1, "#7A8390"), TextBlock("6", 2)),
+    Column(0, 6, TextBlock("考试科目", 1, "#7A8390"), TextBlock("3", 2)),
+    Column(0, 6, TextBlock("待复核", 1, "#7A8390"), ui.textOf(gradeCenter, "pending", 2)))
+gradesPage = Page()
+gradesPage.title = "学生成绩管理中心"
+gradesPage.render(Column(32, 20,
+    TextBlock("学生成绩管理中心", 2, "#273444"),
+    TextBlock("高一（3）班 · 2026 秋季阶段测评 · 虚构示例数据", 1, "#7A8390"),
+    gradeSummary,
+    Row(0, 10, showAllGrades, showAttentionGrades, reviewGrade, resetGrades),
+    GradePanel(20, 16, gradeHeader, RectBlock(730, 1, 0, "#E5EAF0"), gradeTable),
+    ui.textOf(gradeCenter, "message", 1, "#526C83")))