feat(ios): P1-B — W7 UI wave: deep links, timeline, quick-reply, diff, projects, session switcher, thumbnails, lock-screen push

T-iOS-22: DeepLinkRouter (full-field whitelist, cold-start stash, route(from:) for push-tap reuse), 21 tests
T-iOS-24: TimelineSheet mirroring web render() order; disabled→empty-state; reuses AwayDigest onExpand
T-iOS-25: QuickReply chips (built-ins mirror quick-reply.ts via KeyByteMap; visible iff live gate && !readOnly)
T-iOS-27: read-only DiffScreen + App-layer DiffFetcher (RO no-Origin; APIClient fold-in noted for T-iOS-38 owner)
T-iOS-26: Projects list/detail — grouping byte-identical to web group keys (prefs-shared collapse state),
prefs-clobber defenses (no blind PUT on empty base; adopt server echo), claude\r bootstrap
T-iOS-23: UnreadLedger + TitleSanitizer (SessionCore, +15 tests), lastOutputAt decode, list-boundary re-sanitize
T-iOS-29: new-in-cwd (untrusted cwd, no bootstrap re-injection) + exited-session reopen; fixes stale-controller
SwiftTerm view bug via .id(controller.id)
T-iOS-28: offscreen SwiftTerm thumbnail pipeline (LRU 32, concurrency gate 2, 256KiB cap, grid clamp)
T-iOS-21: PushRegistrar (WEBTERM_GATE category: Allow=.authenticationRequired, no .foreground) +
NotificationActionHandler (whitelisted payload, token never persisted, bg-task-wrapped POST, 403 fallback)
CRITICAL fix (verify-found boot crash): @Sendable literals on UN completion closures — MainActor-inherited
closures trapped Swift 6 executor check on UN's background queue; boot re-verified (no new crash reports,
permission prompt reached, privacy shade correctly covering during system alert)
Verified: 261 pkg + 247 app + 10 integration tests green; 7/7 semantics checks; Owns audit clean

ios/App/WebTerm/ViewModels/DiffViewModel.swift

@@ -0,0 +1,164 @@
+import Foundation
+import Observation
+import WireProtocol
+
+/// T-iOS-27 · State for the read-only diff viewer (`DiffScreen`).
+///
+/// The fetch closure is injected —— 生产由 `forProject` 用 `DiffFetcher` 包一
+/// 层（App 装配缝，T-iOS-26 的 ProjectDetail 入口按 (endpoint, path) 构造）；
+/// 测试注入 fake。服务器数据的不可信处理（宽容解码、错误分类）已在
+/// `DiffFetcher` 完成；本 VM 只区分用户可见的四种结局：
+/// - 非空 files → `.loaded`：文件头/hunk 头/行**平铺**为惰性列表行模型
+///   （巨 diff —— 服务器上限 DIFF_MAX_BYTES 2MB —— 绝不合成单个 Text 块）；
+/// - `[]` → `.empty`（truncated 位透传，截断到空也要提示）；
+/// - 400/非法请求 → `.failed(.pathInvalid)`、404 → `.failed(.notFound)`、
+///   其余 → `.failed(.unavailable)` —— 全部可经 `load()` 重试；
+/// - staged/unstaged 切换（`setStaged`）→ 以新范围重新 fetch，同值 no-op。
+@MainActor
+@Observable
+final class DiffViewModel {
+    /// 用户可见的失败三分类（文案映射在 DiffScreen 的 DiffCopy）。
+    enum Failure: Equatable {
+        case pathInvalid
+        case notFound
+        case unavailable
+    }
+
+    /// Rendering phase — an explicit enum so the screen can never show an
+    /// error and diff rows at the same time (same discipline as Timeline).
+    enum Phase: Equatable {
+        case loading
+        /// 服务器回了空 files（该范围无改动）。truncated 位仍需提示。
+        case empty(truncated: Bool)
+        case loaded(DiffPresentation)
+        case failed(Failure)
+    }
+
+    private(set) var phase: Phase = .loading
+    /// 当前范围：false = 工作区（unstaged），true = 已暂存（--staged）。
+    private(set) var staged = false
+
+    @ObservationIgnored
+    private let fetch: @Sendable (_ staged: Bool) async throws -> DiffResult
+
+    init(fetch: @escaping @Sendable (_ staged: Bool) async throws -> DiffResult) {
+        self.fetch = fetch
+    }
+
+    /// 生产装配缝：`DiffScreen(endpoint:path:http:)` 经此构造（T-iOS-26 的
+    /// ProjectDetail 入口只需转手这三样，无需触碰 DiffFetcher）。
+    static func forProject(
+        endpoint: HostEndpoint, path: String, http: any HTTPTransport
+    ) -> DiffViewModel {
+        let fetcher = DiffFetcher(endpoint: endpoint, http: http)
+        return DiffViewModel(fetch: { staged in
+            try await fetcher.fetch(path: path, staged: staged)
+        })
+    }
+
+    /// Fetch and present. Also the「重试」path: callable again from `.failed`.
+    func load() async {
+        phase = .loading
+        do {
+            let result = try await fetch(staged)
+            phase = Self.presentation(for: result)
+        } catch let error as DiffFetchError {
+            phase = .failed(Self.failure(for: error))
+        } catch {
+            phase = .failed(.unavailable) // 传输层等其余错误：可重试兜底
+        }
+    }
+
+    /// staged/unstaged 切换 → 重新 fetch；同值绝不重复请求（任务 Steps）。
+    func setStaged(_ newValue: Bool) async {
+        guard newValue != staged else { return }
+        staged = newValue
+        await load()
+    }
+
+    // MARK: - 纯呈现归约（静态，可单测）
+
+    static func presentation(for result: DiffResult) -> Phase {
+        guard !result.files.isEmpty else {
+            return .empty(truncated: result.truncated)
+        }
+        return .loaded(DiffPresentation(
+            rows: makeRows(files: result.files), truncated: result.truncated
+        ))
+    }
+
+    /// 平铺：文件头 → (binary 占位 | hunk 头 → 行…)…，逐文件顺序保持服务器
+    /// 返回顺序；binary 短路 hunks（镜像 web renderDiffFile 的 early return，
+    /// public/diff.ts:163-166）。id 为稳定递增序号（行内容可重复，不能当身份）。
+    static func makeRows(files: [DiffFile]) -> [DiffRow] {
+        var rows: [DiffRow] = []
+        for file in files {
+            rows.append(DiffRow(id: rows.count, kind: .fileHeader(header(for: file))))
+            if file.binary {
+                rows.append(DiffRow(id: rows.count, kind: .binaryNotice))
+                continue
+            }
+            for hunk in file.hunks {
+                rows.append(DiffRow(id: rows.count, kind: .hunkHeader(hunk.header)))
+                for line in hunk.lines {
+                    rows.append(DiffRow(
+                        id: rows.count, kind: .line(kind: line.kind, text: line.text)
+                    ))
+                }
+            }
+        }
+        return rows
+    }
+
+    private static func failure(for error: DiffFetchError) -> Failure {
+        switch error {
+        case .invalidRequest, .pathInvalid:
+            return .pathInvalid
+        case .projectNotFound:
+            return .notFound
+        case .invalidResponse, .unexpectedStatus:
+            return .unavailable
+        }
+    }
+
+    /// Rename 显示 “old → new”（镜像 web，public/diff.ts:146-150），其余显示
+    /// newPath。路径是服务器字节 —— 屏幕侧一律 `Text(verbatim:)`。
+    private static func header(for file: DiffFile) -> DiffFileHeader {
+        let pathLabel = file.status == .renamed && file.oldPath != file.newPath
+            ? "\(file.oldPath) → \(file.newPath)"
+            : file.newPath
+        return DiffFileHeader(
+            pathLabel: pathLabel, added: file.added,
+            removed: file.removed, status: file.status
+        )
+    }
+}
+
+// MARK: - 行模型（惰性列表的最小呈现单元）
+
+/// Display-ready flattened diff（不可变快照）。
+struct DiffPresentation: Equatable {
+    let rows: [DiffRow]
+    let truncated: Bool
+}
+
+/// One lazy-list row. `id` = 平铺序号（稳定、唯一——文本内容可重复）。
+struct DiffRow: Equatable, Identifiable {
+    enum Kind: Equatable {
+        case fileHeader(DiffFileHeader)
+        case binaryNotice
+        case hunkHeader(String)
+        case line(kind: DiffLineKind, text: String)
+    }
+
+    let id: Int
+    let kind: Kind
+}
+
+/// File-header row payload（路径标签已含 rename 箭头）。
+struct DiffFileHeader: Equatable {
+    let pathLabel: String
+    let added: Int
+    let removed: Int
+    let status: DiffFileStatus
+}