feat(webui): three per-turn diff endpoints and their client wrapper

Ticket 80's plan B, batch 2. The runtime already records what each turn
changed — real per-file line counts, and whether that turn can still be
changed — under the msg_id of the turn's last assistant message. Batch 1
put that id on the wire; nothing read it. These endpoints read it.

GET  /api/turn-diff
POST /api/turn-diff/revert
POST /api/turn-diff/revert is a real write: the engine verifies each file
against the snapshot it captured and then writes or removes it. POST
/api/turn-diff/reapply puts the turn back.

Zero new backend. All three are thin projections over
`applications.session.diff`, reached through the same catalogue host
routes/plugins.js uses. The route reaches exactly that leaf: the
`applications` tree also carries `session.lifecycle`, which can delete a
session, so handing out the handle would hand out deletion too. A
tripwire asserts the only member ever dereferenced is `session`, and that
the route never mentions a gate of its own — auth, rate limiting and
read-only mode are inherited from lib/gates.js.

The engine's selector degrades to "this session's latest turn" when it is
given no id, so a request that lost the coordinate would answer with
ANOTHER turn's counts and its undo button would rewrite that turn's
files. A request without `assistantMessageId` therefore never reaches
the engine at all: it answers `{ok:true,turnDiff:null}`. An id the engine
does not know is a different case and is passed through verbatim, so the
engine — not the route — decides it, and its empty answer stays empty.
A malformed sessionId is a 400. The tests assert the call count, not just
the body, because only the call count catches a "helpful" default.

The engine's 409 for a non-latest turn is passed through with its own
sentence, because that sentence is the only thing that says why an undo
did nothing.

What a successful mutation does NOT do is guess at refreshing the
browser. The response carries the status and the canUndo / canReapply
flips; the client's re-reads are a separate slice.

Author: s39-dev

docs/webui.md

@@ -1532,53 +1532,118 @@ re-arranging the DOM.
 - Neither the `data-active` forced-open rule nor a group's own folding
   semantics change: a collapse intent never overrides forced open.
 
-## The 「已编辑 N 个文件」 card (ticket 77)
+## The 「已编辑 N 个文件」 card (tickets 77 + 83)
 
 The last block of a turn is the edited-files card — the one in the
-desktop screenshot `06-browser-tree-tasks-review.jpg`. It mounts in
-`chat.tsx` after `MessageActions`, matching the reference's mount point
-at `other-minimax-code/.../AssistantBody.tsx:340-380`. The header reuses
-the existing `activity.editedFiles` key; **no new i18n key was added**.
-The activity-group summary and this card are the same sentence, so they
-have to stay the same sentence.
+desktop screenshot `06-browser-tree-tasks-review.jpg`. Ticket 77 mounted
+it once, at the end of the transcript, from the file paths the turn's
+edit tools named, and recorded in this document that the desktop's line
+counts and undo button were unreachable. **Both became reachable in
+ticket 83**, and this section now describes what the card does with
+them.
+
+The header reuses the existing `activity.editedFiles` key for its
+sentence; ticket 83 added keys only for the new affordances (undo, redo,
+the failure copy). The activity-group summary and this card are the same
+sentence, so they have to stay the same sentence.
 
 ### Where the data comes from
 
-The three-tier fallback chain from ticket 61 §4.2 does not resolve at
-tier 1. The card lands on tier 2, but with better semantics than tier 2
-was sketched for: the source is not "the workspace's git changes" but
-**the file paths the turn's edit tools actually named in the transcript**,
-which is turn-scoped by construction.
+Two sources, in strict order of authority, and never merged:
 
-| Tier | Source | Finding |
+| Source | What it is | What it can prove |
 |---|---|---|
-| 1 — turn changeset | `SessionDiffApplication.getTurnDiff` | **Unreachable.** `local-runtime-v2` really does have it (`application/session/diff-application.ts:51`, backed by v1's `LocalTurnDiffCapability` over `SqliteLocalTurnDiffStore`, carrying per-file `additions`/`deletions` and `undo` snapshots). The blocker is addressing, not existence: the transcript is a flat array of glyph-prefixed strings carrying neither `turnId` nor `assistantMessageId`, and `findTurnDiffRecord` (`local-runtime/src/turns/diff-api.ts:209-220`) falls back to `latestForSession` when no selector is supplied — so every historical assistant message would render the same "last turn" card. Separately, `server/lib/runtime-host.js:176-178` forwards only `cliService` / `apiHost` / `controller` and drops the `application` the v2 host returns. Closing either gap means touching `local-runtime-v2`, which ticket 61 §6 risk 1 already reserves for a separate decision. |
-| 2 — the turn's file paths | the transcript's `file-edit` tool blocks | **Adopted.** `collectEditedFiles` (`webapp/lib/edited-files.ts`) walks every `RenderUnit`, reads `toolPaths` off the blocks `isFileEditTool` accepts, normalises separators, de-duplicates, and keeps first-seen order. |
-| 3 — nothing changed | the list is empty | **The card is not rendered at all** (A1). With no file to name there is no empty shell and no 「已编辑 0 个文件」 placeholder. |
+| **The engine's record** | `GET /api/turn-diff` with the turn's `assistantMessageId` | The real per-file `+N` / `-N`, the real file list (which includes edits made through tools whose arguments name no path), and the engine's own `canUndo` / `canReapply` |
+| **The transcript scan** | `collectTurnEditedFiles` (`webapp/lib/edited-files.ts`) over the turn's `RenderUnit`s | Only the file paths a `file-edit` tool named. No counts, no gates. |
+
+The engine's list REPLACES the scan rather than merging with it. A merge
+would double-count a file the two name differently, and the counts would
+then sit on the wrong row. When there is no record — a session recorded
+before the turn coordinate shipped, a legacy transcript read, the `exec`
+transport — the scan is the whole story and the card is exactly what
+ticket 77 shipped.
+
+### The turn coordinate
+
+The engine persists a turn's record under the msg_id of that turn's
+**last assistant message** (`local-runtime-v2/.../turn-outcome.ts` reads
+the last agent *message* response; a turn carries more than one id on
+the wire, one per message segment). The ACP transport already delivered
+that id; the server used to drop it.
+
+- **Live**: `acp.mjs#prompt` keeps the last `agent_message_chunk`
+  `messageId`; `mcode-acp.js#finalize` writes `§§ turn_msg=<id>` next to
+  the `§§ processed_duration=Nms` marker it already wrote. The `§§`
+  family is the established convention for server-written per-turn
+  metadata — third reuse, not a new grammar.
+- **Restored**: `server/lib/transcript.js`'s v2 probe now selects the
+  `turn_id` and `msg_id` columns the message table already carries, and
+  synthesises the same marker from the last assistant row of each turn.
+  Switching away and back gives an existing session its coordinates
+  without asking the engine anything.
+
+`decodeTranscript` consumes the marker and hangs the id on the turn's
+LAST assistant block. A transcript without the marker decodes exactly as
+it did before — every session older than the marker has to keep
+rendering, and a decoder that assumed the marker would throw on all of
+them.
 
 ### What each desktop element maps to
 
 | Desktop element | Here | Why |
 |---|---|---|
 | Header glyph | **Present** | The `pencil` icon from the existing `icons.tsx` set. |
-| 「已编辑 N 个文件」 | **Present** | N counts **distinct files**, not edit calls — a file edited five times is still one file. |
-| Green `+N` / red `-N` | **Absent** | The transcript carries no line statistics and there is no reachable turn-scoped source. Drawing one would be a fabricated number. |
-| 「撤销」 (undo) | **Absent** | No turn-scoped revert endpoint exists; the engine's `rewindTurnDiff` is a side effect of rewinding history, not a per-card undo. A dead button is worse than no button. |
-| 「Review」 | **Absent — the file rows replace it** | A file row opens the real preview through the existing `onOpenFile` chain (red line 4). That is the affordance this codebase can actually honour. |
+| 「已编辑 N 个文件」 | **Present** | N counts **distinct files**, not edit calls. With a record it is the engine's own file count; without one it is the count of paths the scan could name. |
+| Green `+N` / red `-N` | **Present, with a record** | The engine's own per-turn counts, summed in the header. **Absent without one** — a card showing `+0 -0` would say "this file did not change", which is a different and false claim. A zero side is not drawn as `+0` / `-0` either. |
+| 「撤销」 (undo) | **Present, gated on `canUndo`** | Calls `POST /api/turn-diff/revert`, which rewrites real workspace files. |
+| 「重做」 (redo) | **Present, gated on `canReapply`** | Calls `POST /api/turn-diff/reapply`. Appears independently of undo: a reverted turn offers redo and no undo. |
+| 「Review」 | **Absent — the file rows replace it** | A file row opens the real preview through the existing `onOpenFile` chain (red line 4). |
 | File row: type icon + name | **Present** | A `file` icon plus the path's trailing segment; the full path rides on `title` and `data-file-path`. |
-| Per-row added/deleted | **Absent** | Same reason as the header badge. |
+| Per-row added/deleted | **Present, with a record** | The engine's per-file counts, not the header total repeated. |
 | Collapse (3 rows, then expand) | **Present** | A pure client-side state machine, `reduceEditedFilesCardState`; the toggle appears only past three rows. |
 
+The two buttons are **not** greyed out when they cannot act; they are
+absent. Only the latest turn diff can be changed — the engine answers
+`canUndo:false` before the user clicks, and 409 `TURN_DIFF_CONFLICT`
+if they click anyway — so a disabled button would promise an action the
+engine has already refused. The card never re-derives "is this the last
+turn?" from the transcript; the whole coordinate system exists because
+that inference cannot be made safely.
+
+### What a successful undo refreshes
+
+A revert rewrites files the browser is already showing, so five things
+move, in this order:
+
+1. the server's session-tree cache (`invalidateSessionTree()`);
+2. a `session-tree-changed` broadcast (the sidebar);
+3. a `workspace-files-changed` broadcast — a new named SSE frame, no
+   payload, the only signal the webui has that files on disk moved;
+4. on that frame, the files tree re-reads every directory it has open
+   and the git panel re-reads status and branches;
+5. on the same frame, the open file preview re-reads through the refresh
+   path — scroll position preserved, a deleted file shown as a named
+   banner rather than a blank pane, and a **dirty draft left alone**,
+   because its baseline is what a later save conflict-checks against.
+
 ### Contract notes
 
 - The collapse state is **not** persisted in `localStorage`: a reload
   returns to collapsed. It is derived state, not a user preference, so
   red line 3 is untouched.
-- The card derives from the **full** `units` list, not the virtualised
+- Cards are derived from the **full** `units` list, not the virtualised
   `visibleUnits` window — otherwise scrolling past 200 units would make
-  it blink in and out.
+  them blink in and out. Each turn's card sits at **its own** turn's last
+  unit, so a three-turn session reads as three cards; the final turn's
+  card stays at the transcript tail, after the message-action row, which
+  is where ticket 77 put the single card and where the desktop puts it.
 - With no `onOpenFile` wired, a file row degrades to plain text rather
   than to a button that does nothing.
+- The route exposes `applications.session.diff` and nothing else; the
+  `applications` tree also carries `session.lifecycle`, which can delete
+  a session.
+- `previewState` is declared by the protocol and is always `undefined` on
+  this path. Nothing renders it.
 
 ## Loading states: transcript skeleton and streaming indicator (ticket U8)
 

docs/webui.zh-CN.md

@@ -1103,44 +1103,92 @@ loading-states 相同：让 SSR 渲染测试可以脱离 `chat.tsx` 的 `@/` 别
 - `data-active` 强制展开规则与活动组自身的折叠语义都不因此改变：
   收起意图不会盖过强制展开。
 
-## 「已编辑 N 个文件」汇总卡（工单 77）
+## 「已编辑 N 个文件」汇总卡（工单 77 + 83）
 
 回合的最后一块是这张卡（桌面版 `06-browser-tree-tasks-review.jpg`
-里的那张），挂载点在 `chat.tsx` 的 `MessageActions` 之后——与
-参照仓 `AssistantBody.tsx:340-380` 的位置一致。文案复用既有的
-`activity.editedFiles` 键，**没有新增 i18n 键**：活动组摘要和这
-张卡说的是同一句话，就该是同一个键。
+里的那张）。工单 77 把它挂在转录尾部，数据源是编辑类工具点名的文件
+路径，并在本文档里记录了桌面版的行数统计与撤销按钮「不可达」。**两者
+在工单 83 变得可达**，本节现在描述卡片拿到它们之后的行为。
 
-### 数据从哪来（三级降级链，审计结论）
+表头那句文案仍复用 `activity.editedFiles` 键；工单 83 只为新增的
+能力（撤销、重做、失败文案）新增了键。活动组摘要和这张卡说的是同一
+句话，就该是同一个键。
 
-工单 61 §4.2 定的降级链，第一级审计**没通过**。卡片落在第 2 级，
-但语义比第 2 级原设想更好：数据源不是「工作区 git 变更」而是
-**转录里编辑类工具真实点名的文件路径**，天然是回合级的。
+### 数据从哪来
 
-| 级 | 数据源 | 审计结果 |
+两个来源，权威性有严格先后，且**从不合并**：
+
+| 来源 | 是什么 | 能证明什么 |
 |---|---|---|
-| 1 回合级 changeset | `SessionDiffApplication.getTurnDiff` | **不可达**。`local-runtime-v2` 确实有这套能力（`application/session/diff-application.ts:51`，由 v1 的 `LocalTurnDiffCapability` 落到 `SqliteLocalTurnDiffStore`，逐文件带 `additions`/`deletions` 与 `undo` 快照），但 webui 拿不到回合坐标：转录是字形前缀的纯字符串数组，不携带 `turnId` 也不携带 `assistantMessageId`，而 `findTurnDiffRecord`（`local-runtime/src/turns/diff-api.ts:209-220`）在没有选择器时退化成 `latestForSession`——挂在每条历史助手消息下都是同一张「最后一回合」的卡。另外 webui 侧 `server/lib/runtime-host.js:176-178` 只回传 `cliService`/`apiHost`/`controller`，把 v2 宿主返回的 `application` 丢掉了。补齐要动 `local-runtime-v2`，工单 61 第六节风险 1 已声明需另行拍板。 |
-| 2 回合级文件路径 | 转录的 `file-edit` 工具块 | **采用**。`collectEditedFiles`（`webapp/lib/edited-files.ts`）走遍 `RenderUnit[]`，对 `isFileEditTool` 为真的工具块取 `toolPaths`，按分隔符归一去重，保留首次出现顺序。 |
-| 3 无变更 | 列表为空 | **整卡不渲染**（A1）。没有文件可点名时不画空壳、不画「已编辑 0 个文件」。 |
+| **引擎记录** | 带该回合 `assistantMessageId` 的 `GET /api/turn-diff` | 真实的逐文件 `+N` / `-N`、真实的文件清单（含参数里没写路径的工具改的文件），以及引擎自己的 `canUndo` / `canReapply` |
+| **转录扫描** | `collectTurnEditedFiles`（`webapp/lib/edited-files.ts`）在该回合的 `RenderUnit[]` 上扫描 | 只有 `file-edit` 工具点名的文件路径。没有行数，没有门控 |
+
+引擎清单**取代**扫描，而不是与之合并。合并会让两个来源命名不同的
+同一文件被数两次，计数随后落在错误的行上。没有记录时——回合坐标
+上线之前录的会话、legacy 转录回读、`exec` 传输——扫描就是全部，卡片
+也就是工单 77 交付的那张。
+
+### 回合坐标
+
+引擎把一回合的记录存在该回合**最后一条助手消息**的 msg_id 下
+（`local-runtime-v2/.../turn-outcome.ts` 读的是最后一条 agent
+*message* 应答；线上一个回合不止一个 id，每个消息段一个）。ACP
+传输早就把这个 id 送到了服务端，此前被丢掉。
+
+- **实时路径**：`acp.mjs#prompt` 留住最后一个 `agent_message_chunk`
+  的 `messageId`；`mcode-acp.js#finalize` 在它本来就写的
+  `§§ processed_duration=Nms` 旁边写下 `§§ turn_msg=<id>`。`§§`
+  这一族是服务端回合元数据的既有约定——这是第三次复用，不是新语法。
+- **回读路径**：`server/lib/transcript.js` 的 v2 探针现在 SELECT
+  消息表本来就有的 `turn_id` 与 `msg_id` 两列，用每个回合最后一条
+  助手行合成同一款标记。切走再切回，老会话就拿到了坐标，不必向引擎
+  要任何东西。
+
+`decodeTranscript` 吞掉标记，把 id 挂在该回合**最后**一个助手块上。
+没有标记的转录解码结果与之前逐块相同——早于标记的每个会话都得继续
+正常渲染，而假设标记一定存在的解码器会把它们全部解崩。
 
 ### 桌面版每个元素对应什么
 
 | 桌面版元素 | 我们的对应 | 说明 |
 |---|---|---|
 | 头部图标 | **有** | `pencil` 图标，沿用 `icons.tsx` 既有图标集。 |
-| 「已编辑 N 个文件」 | **有** | N 是**去重后的文件数**，不是编辑调用次数——同一文件改五次仍算一个文件。 |
-| 绿色 `+N` / 红色 `-N` 增删统计 | **没有** | 转录不含任何行数统计，本仓也没有可达的回合级行数来源。画一个就是编数字。 |
-| 「撤销」按钮 | **没有** | 没有回合级回滚端点；引擎的 `rewindTurnDiff` 是历史回退的副作用，不是按卡片撤销。不画死按钮。 |
-| 「Review」按钮 | **没有（以文件行替代）** | 点文件行走既有的 `onOpenFile` 链路开文件预览（红线 4），这是本仓真能兑现的动作。 |
+| 「已编辑 N 个文件」 | **有** | N 是**去重后的文件数**。有记录时是引擎自己的文件数；没有时是扫描能点名的数量。 |
+| 绿色 `+N` / 红色 `-N` | **有记录时有** | 引擎自己的逐回合计数，表头求和。**没有记录时不画**——画 `+0 -0` 等于宣称「这个文件没变」，那是另一句且为假的话。某一边为 0 时也不画 `+0` / `-0`。 |
+| 「撤销」按钮 | **有，由 `canUndo` 门控** | 调 `POST /api/turn-diff/revert`，真的会改写工作区文件。 |
+| 「重做」按钮 | **有，由 `canReapply` 门控** | 调 `POST /api/turn-diff/reapply`。与撤销独立出现：已撤销的回合给重做、不给撤销。 |
+| 「Review」按钮 | **没有（以文件行替代）** | 点文件行走既有的 `onOpenFile` 链路开文件预览（红线 4）。 |
 | 文件行：类型图标 + 文件名 | **有** | `file` 图标 + 路径末段；完整路径挂在 `title` 与 `data-file-path` 上。 |
-| 文件行右侧增删数 | **没有** | 同增删统计。 |
+| 文件行右侧增删数 | **有记录时有** | 是该文件自己的计数，不是把表头总数重复一遍。 |
 | 折叠（先 3 行 + 展开） | **有** | 纯客户端状态机 `reduceEditedFilesCardState`，超过 3 行才出现切换。 |
 
+两个按钮在不能动作时是**不画**，不是灰掉。只有最新回合的 diff 能改
+——引擎会在用户点击之前就回 `canUndo:false`，真点了也回 409
+`TURN_DIFF_CONFLICT`——所以一个禁用的按钮等于承诺一件引擎已经拒绝
+的事。卡片从不从转录反推「这是不是最后一回合」；整套坐标方案存在的
+理由就是这个推断做不安全。
+
+### 一次成功撤销要刷新什么
+
+撤销改写的是浏览器正在显示的文件，因此有五步，按此顺序：
+
+1. 服务端的会话树缓存（`invalidateSessionTree()`）；
+2. 一次 `session-tree-changed` 广播（侧边栏）；
+3. 一次 `workspace-files-changed` 广播——新的命名 SSE 帧，无载荷，
+   是 webui 唯一能得知「磁盘上的文件在自己脚下变了」的信号；
+4. 收到该帧后，文件树重读所有已展开的目录，git 面板重读 status 与
+   branches；
+5. 同一帧触发，文件预览走刷新通道重读当前文件——保留滚动位置；文件
+   已删时给具名提示条而不是空白面板；**有未保存草稿则不动**，因为那份
+   草稿的基线正是后续保存要冲突校验的对象。
+
 ### 契约要点
 
 - 折叠状态**不进 `localStorage`**：刷新后回到折叠态。这是派生状态而非用户偏好，红线 3 不受影响。
-- 卡片从**完整** `units` 派生，不是虚拟窗口 `visibleUnits`——否则超过 200 个单元后滚动会让它忽隐忽现。
+- 卡片从**完整** `units` 派生，不是虚拟窗口 `visibleUnits`——否则超过 200 个单元后滚动会让它们忽隐忽现。每个回合的卡挂在**该回合**最后一个单元后，所以三个回合读起来就是三张卡；最后一个回合的卡仍在转录尾部、在消息操作行之后——那是工单 77 放单张卡的位置，也是桌面版的位置。
 - 未接 `onOpenFile` 时文件行降级为纯文本，**不退化成点了没反应的按钮**。
+- 路由只透出 `applications.session.diff`，不多给一个成员——同一棵 `applications` 树上还有能删会话的 `session.lifecycle`。
+- `previewState` 协议里有定义，这条链路上恒为 `undefined`，任何地方都不渲染它。
 
 ## 加载态：会话骨架屏与流式活动指示（工单 U8）