补强传播帧清空识别

- 按帧范围清空时检查完整选择帧段，确保范围内人工/AI 标注帧会触发确认
- 将 propagation_seed_signature 和 legacy propagation source 纳入传播 mask 识别
- 同步 Canvas、时间轴和工作区的传播帧判断，避免传播帧误判为人工帧
- 更新项目指南、需求冻结和设计冻结文档中的传播链识别字段

src/components/VideoWorkspace.tsx

@@ -261,12 +261,13 @@ const propagationHistoryEqual = (
 );
 
 const isPropagatedMask = (mask: Mask) => {
-  const source = typeof mask.metadata?.source === 'string' ? mask.metadata.source : '';
-  return source.includes('_propagation')
+  const source = typeof mask.metadata?.source === 'string' ? mask.metadata.source.toLowerCase() : '';
+  return source.includes('propagat')
     || mask.metadata?.propagated_from_frame_id !== undefined
     || mask.metadata?.source_annotation_id !== undefined
     || mask.metadata?.source_mask_id !== undefined
-    || mask.metadata?.propagation_seed_key !== undefined;
+    || mask.metadata?.propagation_seed_key !== undefined
+    || mask.metadata?.propagation_seed_signature !== undefined;
 };
 
 const metadataNumber = (value: unknown): number | null => {