feat(capcut): 合并同类型轨道并优化音频/字幕边界裁剪

在同步到本地剪映后新增轨道合并功能，将因降级添加导致的多条同类型轨道合并为单条。同时修复音频和字幕添加时可能跨场景重叠的问题，将 end 裁剪到 slot 边界。

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

@@ -25,6 +25,7 @@ const {
   addImages, addVideos, addSlotsLocally,
   addVoiceover, addBGM,
   addSubtitles,
+  consolidateTracks,
   addEffects, addFilter,
 } = require('./lib/capcut-tracks')
 const { saveManifest } = require('./lib/pipeline-utils')
@@ -229,7 +230,7 @@ async function assemble(args) {
 
   const steps = []
   if (mode === 'images') steps.push('upload')
-  steps.push('draft', 'materials', 'kenburns', 'audio_oss', 'voiceover', 'audio', 'subtitles', 'effects', 'filter', 'save', 'sync')
+  steps.push('draft', 'materials', 'kenburns', 'audio_oss', 'voiceover', 'audio', 'subtitles', 'effects', 'filter', 'save', 'sync', 'consolidate')
   const totalSteps = steps.length
   let step = 0
 
@@ -407,6 +408,10 @@ async function assemble(args) {
   await syncToLocalJianying(draftUrl, draftId, totalDurationUs)
   console.log('   同步完成\n')
 
+  // -- 合并同类型轨道（TTS 逐条降级时每条独占一个轨道）--
+  step++; console.log(`[${step}/${totalSteps}] 合并同类型轨道...`)
+  consolidateTracks(draftId)
+
   // -- 视频轨道 slot 写入（在 syncToLocalJianying 之后执行，此时本地草稿文件已存在）--
   if (mode !== 'images') {
     step++; console.log(`[${step}/${totalSteps}] 写入视频轨道时间线...`)