feat(video-pipeline): 将素材过渡动画切换为闪白并缩短时长

将图片和视频素材的默认过渡效果从溶解改为闪白，并将过渡时长从300ms缩短至150ms，以提升视频拼接的视觉流畅度。

.claude/skills/video-from-script/scripts/capcut_assemble.js

@@ -432,8 +432,8 @@ async function addImages(draftUrl, items, imgUrls, timeline, width, height, anim
       end: tl.end,
       duration: tl.duration,
       animation: animation || '',
-      transition: i > 0 ? '溶解' : '',
-      transition_duration: 300000,
+      transition: i > 0 ? '闪白' : '',
+      transition_duration: 150000,
     }
   })
 
@@ -520,8 +520,8 @@ async function addVideos(draftUrl, inputDir, items, timeline, width, height) {
       end: tl.end,
       duration: tl.duration,
       mask: '',
-      transition: i > 0 ? '溶解' : '',
-      transition_duration: 300000,
+      transition: i > 0 ? '闪白' : '',
+      transition_duration: 150000,
       volume: item.volume || 1,
     }
   })