feat(video-pipeline): 用 ffprobe 实际测量音视频时长并统一字段名为 script

将项目中的 `narration` 字段统一重命名为 `script`，并新增 `getAudioDurationSec` 函数通过 `ffprobe` 实际测量音频和视频文件的时长，替代 Manifest 中的估计值，提高时间线组装的准确性。同时优化字幕逻辑，仅在有 TTS 音频时调整视频速度。

.claude/skills/video-from-script/scripts/lib/phase-tts.js

@@ -15,7 +15,7 @@ async function phaseTts(manifest, manifestPath, options = {}) {
   const { synthesize } = require('../qwen-tts')
 
   const items = manifest.items.filter(it =>
-    it.status === 'done' && (it.narration || it.text) && !it.audio
+    it.status === 'done' && (it.script || it.text) && !it.audio
   )
   if (items.length === 0) { log('tts', '无待处理 item，跳过'); return }
 
@@ -25,7 +25,7 @@ async function phaseTts(manifest, manifestPath, options = {}) {
     const item = items[i]
     const idx = i + 1
     try {
-      const { filePath, duration } = await synthesize(item.narration || item.text, {
+      const { filePath, duration } = await synthesize(item.script || item.text, {
         outputDir: audioDir,
         id: item.id || idx,
         voice: manifest.ttsVoice || undefined,
@@ -33,7 +33,7 @@ async function phaseTts(manifest, manifestPath, options = {}) {
       })
       item.audio = path.relative(dir, filePath).replace(/\\/g, '/')
       item.audioDuration = Math.round(duration * 1000) / 1000
-      log('tts', `[${idx}/${items.length}] ${duration.toFixed(1)}s: ${(item.narration || item.text).substring(0, 30)}...`)
+      log('tts', `[${idx}/${items.length}] ${duration.toFixed(1)}s: ${(item.script || item.text).substring(0, 30)}...`)
     } catch (err) {
       item.status = 'failed'
       item.error = `TTS失败: ${err.message}`