feat: 为图片遮罩新增 KenBurns 效果及视频阶段并发优化

- capcut_assemble: 添加 KenBurns 效果支持（慢速推拉动画）
- phase-videos: 区分"无任务"与"已有 video"的日志输出，避免误报跳过
- 张不苦账号提示词文档精简（拆分/图片提词/遮罩提词词）
- 新增 CHANGELOG.md 和 CONSTRAINT_ENGINE.md 约束引擎文档

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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

@@ -27,6 +27,7 @@ const {
   addSubtitles,
   consolidateTracks,
   addEffects, addFilter,
+  addKenBurns,
 } = require('./lib/capcut-tracks')
 const { saveManifest } = require('./lib/pipeline-utils')
 const { syncDraft, registerDraft } = require('./sync-to-jianying')

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

@@ -39,7 +39,14 @@ async function phaseVideos(manifest, manifestPath, options) {
     }
     items.push(it)
   }
-  if (items.length === 0) { log('videos', '无待处理 item，跳过'); return }
+
+  const pendingItems = items.filter(it => !it.video)
+  if (pendingItems.length === 0) {
+    // 有 videoCandidates 但全部已有 video → 直接返回（不打印跳过消息）
+    return
+  }
+
+  log('videos', `共 ${pendingItems.length} 个待处理`)
 
   // 选择生成器
   let Api, pollFn
@@ -103,7 +110,7 @@ async function phaseVideos(manifest, manifestPath, options) {
 
   // Phase 2: 提交新任务（并发 3）
   const concurrency = 3
-  log('videos', `提交 ${needSubmit.length} 个新任务（并发: ${concurrency}）...`)
+  log('videos', `共 ${needSubmit.length} 个新任务（并发: ${concurrency}）...`)
 
   const submitted = []
   for (let i = 0; i < needSubmit.length; i += concurrency) {