fix(canvas): 3D 导演台 - 修复白膜视频导出渲染循环逐帧崩溃 (#439)

导出白膜视频得到的是 0 秒视频：文件实测仅 1 帧（ffprobe
nb_read_frames=1，duration=0.001s），却无任何报错并照常回写画布。

根因：applyClaySceneMaterials 把场景内所有非 actor mesh 的材质换成
clay MeshStandardMaterial，drei Grid 的 GridMaterial（带 uniforms 的
ShaderMaterial）也在其中。Grid 组件内部 useFrame 每帧读取
material.uniforms.worldCamProjPosition，材质被换后 uniforms 为
undefined，渲染循环逐帧抛 TypeError，R3F 不再产出新帧，
captureStream 只录到启动前手动 gl.render 的那一帧；5 秒后
setTimeout 照常 stop，残缺视频被当作成功结果。开启轨迹显示
（drei Line 同为 uniforms 材质）或视口白膜显示模式时同样触发。

修复：
- applyClaySceneMaterials 跳过带 uniforms 的 ShaderMaterial，
  并抽到 lib/canvas/director/director-clay-materials.ts 补回归测试；
- recordCanvas 加固：录制期间监听 window 未捕获错误，首个渲染
  错误立即中止并报错；录制结束用 <video> 探测真实时长（Chrome
  MediaRecorder 的 webm 无时长头，需 seek 到末尾），低于预期一半
  报「时长异常」，杜绝残缺视频再被静默回写。

web/test/director-clay-materials.test.ts

@@ -0,0 +1,56 @@
+import { describe, expect, test } from "bun:test";
+import { Line, LineBasicMaterial, Mesh, MeshBasicMaterial, MeshStandardMaterial, PlaneGeometry, Scene, ShaderMaterial, type Material } from "three";
+
+import { applyClaySceneMaterials } from "../src/lib/canvas/director/director-clay-materials";
+
+const geometry = new PlaneGeometry(1, 1);
+
+describe("导演台白膜材质切换", () => {
+    test("标准材质 mesh 换成 clay，恢复后还原为原材质", () => {
+        const scene = new Scene();
+        const original = new MeshStandardMaterial({ color: "#ff0000" });
+        const mesh = new Mesh(geometry, original);
+        scene.add(mesh);
+        const restore = applyClaySceneMaterials(scene);
+        expect(mesh.material).not.toBe(original);
+        expect(mesh.material).toBeInstanceOf(MeshStandardMaterial);
+        restore();
+        expect(mesh.material).toBe(original);
+    });
+
+    test("带 uniforms 的 shader 材质保持原样（Grid 逐帧崩溃回归）", () => {
+        const scene = new Scene();
+        const shader = new ShaderMaterial({ uniforms: { worldCamProjPosition: { value: null } } });
+        const grid = new Mesh(geometry, shader);
+        scene.add(grid);
+        const restore = applyClaySceneMaterials(scene);
+        expect(grid.material).toBe(shader);
+        restore();
+        expect(grid.material).toBe(shader);
+    });
+
+    test("directorActor mesh 与非 Mesh 对象不受影响", () => {
+        const scene = new Scene();
+        const actorMaterial = new MeshBasicMaterial();
+        const actor = new Mesh(geometry, actorMaterial);
+        actor.userData.directorActor = true;
+        const line = new Line(geometry, new LineBasicMaterial());
+        scene.add(actor, line);
+        const restore = applyClaySceneMaterials(scene);
+        expect(actor.material).toBe(actorMaterial);
+        expect(line.material).toBeInstanceOf(LineBasicMaterial);
+        restore();
+        expect(actor.material).toBe(actorMaterial);
+    });
+
+    test("材质数组混入 uniforms 材质时整组跳过", () => {
+        const scene = new Scene();
+        const materials: Material[] = [new MeshStandardMaterial(), new ShaderMaterial({ uniforms: {} })];
+        const mesh = new Mesh(geometry, materials);
+        scene.add(mesh);
+        const restore = applyClaySceneMaterials(scene);
+        expect(mesh.material).toBe(materials);
+        restore();
+        expect(mesh.material).toBe(materials);
+    });
+});