Lazy-load page textures with LRU cache and double-sided materials

Replace eager texture generation with a reference-counted pool
that loads only the visible window and evicts unused pages, and
swap front/back mesh pairs for single meshes using a two-sided
material to halve draw calls. The timeline now drives its texture
window from the camera's visible world range each frame, and the
binder tracks a current-page window. Flip animations also finalize
immediately so rapid flips chain smoothly.

src/core/App.js

@@ -165,7 +165,7 @@ export class App {
         this.interactives.binder.push(m);
       });
     });
-    this.timeline.cards.forEach((c) => this.interactives.timeline.push(c.front, c.back));
+    this.timeline.cards.forEach((c) => this.interactives.timeline.push(c.mesh));
     // 地理视图由 Cesium 自行拾取，不参与 Three.js 射线检测
   }
 
@@ -258,6 +258,18 @@ export class App {
     return (2 * Math.tan(halfFov) * dist * aspect) / canvas.clientWidth;
   }
 
+  // 时间轴：按当前相机可视区域刷新纹理窗口（拖拽/缩放/居中动画时连续跟随）
+  _updateTimelineTextures() {
+    if (this.view !== 'timeline') return;
+    const dist = this.camera.position.distanceTo(this.controls.target);
+    const halfFov = THREE.MathUtils.degToRad(this.camera.fov / 2);
+    const el = this.renderer.domElement;
+    const aspect = el.clientWidth / el.clientHeight;
+    const halfW = Math.tan(halfFov) * dist * aspect;
+    // 时间轴相机始终在 z 轴看向原点（旋转已锁定），可见世界 x ∈ [-halfW, halfW]
+    this.timeline.setVisibleWorldRange(-halfW, halfW);
+  }
+
   _hitTest() {
     const list = this.interactives[this.view];
     if (!list || !list.length) return null;
@@ -388,6 +400,7 @@ export class App {
       const limit = TIMELINE_SPREAD + 0.6;
       this.timelineOffset = clamp(-this.timeline.getCardX(this.selected), -limit, limit);
       this.timeline.group.position.x = this.timelineOffset;
+      this.timeline.refreshTextures();
     }
 
     if (!isGlobe) {
@@ -444,6 +457,8 @@ export class App {
 
     this.binder.update(dt);
     this.timeline.update(dt);
+    this.timeline.drainPending();
+    this._updateTimelineTextures();
 
     if (this.cameraTween) {
       const t = this.cameraTween;