性能优化：精灵降采样、像素比降低、麂皮法线降分辨率

- 时间轴文字精灵超采样 SS 4→2，显存省 75%
- Three.js 像素比上限 2→1.5，Cesium resolutionScale 1.5→1.0
- 麂皮法线贴图 1024×1280→512×640，启动逐像素计算省 75%

src/core/App.js

@@ -64,7 +64,7 @@ export class App {
   _initRenderer() {
     const canvas = document.getElementById('scene');
     this.renderer = new THREE.WebGLRenderer({ canvas, antialias: true });
-    this.renderer.setPixelRatio(Math.min(window.devicePixelRatio, 2));
+    this.renderer.setPixelRatio(Math.min(window.devicePixelRatio, 1.5));
     this.renderer.setSize(window.innerWidth, window.innerHeight);
     this.renderer.shadowMap.enabled = true;
     this.renderer.shadowMap.type = THREE.PCFSoftShadowMap;