feat(video-from-script): 支持分段音频时长测量

为 `capcut_assemble.js` 中的 item 增加 segments 属性支持，当 item 包含多个音频分段时，计算各分段总时长并赋给 `audioDuration`，提升对分段式音轨素材的处理能力。

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

@@ -177,7 +177,21 @@ async function assemble(args) {
   // 测量实际时长
   let audioMeasured = 0, videoMeasured = 0
   for (const item of items) {
-    if (item.audio && !item.audio.startsWith('http')) {
+    if (item.segments && item.segments.length > 0) {
+      let totalDur = 0
+      for (const seg of item.segments) {
+        if (seg.audio && !seg.error) {
+          const segPath = path.isAbsolute(seg.audio) ? seg.audio : path.resolve(inputDir, seg.audio)
+          if (fs.existsSync(segPath)) {
+            const d = await getAudioDurationSec(segPath)
+            if (d != null) totalDur += d
+          } else if (seg.duration) {
+            totalDur += seg.duration
+          }
+        }
+      }
+      if (totalDur > 0) { item.audioDuration = totalDur; audioMeasured++ }
+    } else if (item.audio && !item.audio.startsWith('http')) {
       const audioPath = path.isAbsolute(item.audio)
         ? item.audio
         : path.resolve(inputDir, item.audio)