修复评审器跨组件修改项跳转丢失并展开完整文档视图

- DiffView 聚焦 effect 依赖补上 before/after：stage 文本到达、
  MergeView 重建后重新应用选区和滚动，跨组件点击不再停在顶部；
- 非可编辑面板加 drawSelection，聚焦范围在浏览器中可见；
- 移除 collapseUnchanged，评审始终可读完整未改动段落；
- 滚动容器移到 .cm-mergeView 外层，编辑器高度自适应内容；
- 新增 3 项 Vitest 覆盖折叠移除与聚焦时序，共 9 项通过；
- README 与 explanation 同步当前检查数量。

reviewer/src/client/DiffView.tsx

@@ -1,7 +1,7 @@
 import { markdown } from "@codemirror/lang-markdown";
 import { MergeView } from "@codemirror/merge";
-import { EditorState } from "@codemirror/state";
-import { EditorView, lineNumbers } from "@codemirror/view";
+import { EditorSelection, EditorState } from "@codemirror/state";
+import { drawSelection, EditorView, lineNumbers } from "@codemirror/view";
 import { useEffect, useRef } from "react";
 
 interface DiffViewProps {
@@ -45,6 +45,7 @@ const readOnlyExtensions = [
   markdown(),
   EditorState.readOnly.of(true),
   EditorView.editable.of(false),
+  drawSelection(),
   EditorView.lineWrapping,
   editorTheme,
 ];
@@ -64,7 +65,6 @@ export function DiffView({ before, after, beforeLabel, afterLabel, focusRange }:
       orientation: "a-b",
       gutter: true,
       highlightChanges: true,
-      collapseUnchanged: { margin: 4, minSize: 8 },
     });
     merge.current = view;
     return () => {
@@ -73,6 +73,8 @@ export function DiffView({ before, after, beforeLabel, afterLabel, focusRange }:
     };
   }, [before, after]);
 
+  // 切换组件时 stage 文本先到、旧视图上的提前滚动会被重建覆盖，所以聚焦
+  // 必须等 MergeView 换成目标文本之后再执行。
   useEffect(() => {
     const view = merge.current;
     if (view === null || focusRange === null || focusRange === undefined) {
@@ -81,11 +83,11 @@ export function DiffView({ before, after, beforeLabel, afterLabel, focusRange }:
     const anchor = Math.min(Math.max(focusRange.start, 0), view.a.state.doc.length);
     const head = Math.min(Math.max(focusRange.end, anchor), view.a.state.doc.length);
     view.a.dispatch({
-      selection: { anchor, head },
+      selection: EditorSelection.range(anchor, head),
       effects: EditorView.scrollIntoView(anchor, { y: "center" }),
     });
     view.a.focus();
-  }, [focusRange]);
+  }, [focusRange, before, after]);
 
   return (
     <section className="diff-shell" aria-label={`${beforeLabel}与${afterLabel}对比`}>