feat(diff): 折叠行异步展开状态机与内容切片

- src/client/api.ts: 仿 gitCommitDiff 新增 gitShow 封装，走既有 git.show 服务路由（payload {rev, path}，返回 content 或 null），服务端零改动
- src/client/diff/rows.ts: 新增 foldRowsFromContents 纯函数——old 侧按 fold 行号切片为 context 行（new 侧经区间偏移映射，落界才带行号），new 侧补纯增行；越界截断、\r 保留
- src/client/diff/DiffRows.tsx: 新增 resolveFold prop；foldData/foldLoading/foldFailed 三态，加载中不可点，失败降级静态标记；foldEpoch 使 segments 重置后 in-flight 回调失效
- src/client/diff/DiffFiles.tsx: resolveFold 下传各文件 DiffRows，fold 级 Promise 缓存去重并发点击与重挂载
- tests/diff-rows.spec.ts: foldRowsFromContents 六用例（对称/纯删/纯增/\r\n/空内容/越界）

背景：git -U3 不输出 hunk 间未改动行，gap fold 只有行号范围没有 rows，渲染复用「点击展开」文案却无交互；展开能力建为独立状态机（含 api 封装），git lens 只需注入取数 loader。
设计 docs/plans/2026-09-07-git-fold-expand-design.md §4.1-4.4。

src/client/api.ts

@@ -316,6 +316,11 @@ export const api = {
   /** Full patch text of one commit (diff display for the history rows). */
   gitCommitDiff: (scope: SessionScope, hash: string, worktree?: string, signal?: AbortSignal) =>
     call<{ diff: string }>('git.commit-diff', gitPayload(scope, worktree, { hash }), signal),
+  /** One file's content at a revision (`git show <rev>:<path>`); null when the
+   *  revision has no such path. The diff views' on-demand hunk-fold expansion
+   *  reads both sides' full contents through this. */
+  gitShow: (scope: SessionScope, rev: string, path: string, worktree?: string, signal?: AbortSignal) =>
+    call<{ content: string | null }>('git.show', gitPayload(scope, worktree, { rev, path }), signal),
   /** The session's file-tool events for the changes tab's session lens: the
    *  `tool/call` + `tool/result` rows past `afterSeq` (0 = whole window),
    *  capped to the recent window host-side. The client runtime exposes no