feat(ai写作): 草稿局部修改只改命中章节，支持改章节标题，修改时右侧流式重写

用户在草稿确认填修改意见(user_revise)时，按章节定位只重写命中章节、
其余字节级保留；新增范围识别区分「改标题」与「改正文」，改标题同步回写
outline 使其持久生效（修复改章节标题不生效）；整体性/结构性意见自动回退
整篇重写。前端新增 isRevising：修改时右侧先恢复初始占位（清空旧稿），
重写流到达后实时展示被改章节的流式重写。

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend-web/src/open-canvas/components/ai-writing/AIWritingDraftPanel.tsx

@@ -41,10 +41,23 @@ export function AIWritingDraftPanel() {
   const showCompletedDraftView = isDraftEditable || isHistoryMode;
 
   const writingStatus = writing?.state.status;
+  const streamingSectionCount = writing?.state.streamingDraftSections.length ?? 0;
+
+  // 按用户意见修改中、且重写流还没开始（无任何流式章节）：右侧先恢复初始空占位，
+  // 把旧稿清掉；一旦重写流式 chunk 到达（streamingDraftSections 非空）就走下面的
+  // 流式预览，实时展示重写过程；draft_ready 后 isRevising 置回 false 显示新成稿。
+  if (writing?.state.isRevising && streamingSectionCount === 0) {
+    return (
+      <div className="h-full bg-muted/10 border-l">
+        <DraftWaitingPlaceholder />
+      </div>
+    );
+  }
+
   // 仅当章节仍在累积时走流式 Markdown；draft_ready 清空 sections 后即使 status 尚未切走也进画布
   const isActivelyStreamingDraft =
     (writingStatus === 'writing_draft' || writingStatus === 'revising') &&
-    (writing?.state.streamingDraftSections.length ?? 0) > 0;
+    streamingSectionCount > 0;
 
   const streamingMarkdown = isActivelyStreamingDraft && writing
     ? assembleStreamingDraftMarkdown(

frontend-web/src/open-canvas/hooks/useAIWritingStream.ts

@@ -70,6 +70,7 @@ const INITIAL_STATE: AIWritingState = {
   streamingReviewItems: [],
   revisionSummary: null,
   pendingRevisionContext: null,
+  isRevising: false,
 };
 
 export { INITIAL_STATE };
@@ -118,7 +119,8 @@ export function reducer(prev: AIWritingState, action: Action): AIWritingState {
         ...(ns === 'writing_outline' ? { outlineStreamingText: '', outlineThinkingText: '' } : {}),
         ...(ns === 'writing_draft' ? { streamingDraftSections: [], streamingDraftTitle: '' } : {}),
         ...(ns === 'reviewing' ? { streamingReviewItems: [] } : {}),
-        ...(ns === 'revising' ? { streamingDraftSections: [], streamingDraftTitle: '' } : {}),
+        // 进入「修改」：右侧画布恢复初始空占位（isRevising），不再显示旧稿/流式重写
+        ...(ns === 'revising' ? { streamingDraftSections: [], streamingDraftTitle: '', isRevising: true } : {}),
         completedInterventions: [...prev.completedInterventions, action.record],
         ...(action.pendingRevisionContext ? { pendingRevisionContext: action.pendingRevisionContext } : {}),
       };
@@ -357,6 +359,8 @@ function handleEvent(prev: AIWritingState, event: ProgressEvent): AIWritingState
         streamingDraftTitle: '',
         revisionSummary: summary,
         pendingRevisionContext: null,
+        // 修改完成：解除右侧空占位，展示新成稿
+        isRevising: false,
       };
     }
 

frontend-web/src/open-canvas/types/ai-writing.ts

@@ -389,4 +389,7 @@ export interface AIWritingState {
     skippedIssues: ReviewIssue[];
     oldWordCount: number;
   } | null;
+  /** 是否正按用户意见修改中（revising 提交起 → draft_ready 止）。
+   *  为 true 时右侧画布恢复初始空占位，不展示旧稿 / 流式重写过程。 */
+  isRevising: boolean;
 }