feat(mobile): ios background upload bridge (compile-gated at G3)

Swift 侧（协议与 Android UploadPlugin.kt 逐字一致，编译/真机留 G3）：
- UploadBridge.swift：通道/方法/事件/失败码常量 + registerTask 参数解析
  （镜像 UploadProtocol.kt）
- UploadEventBus.swift：单调 seq + 终态 ack 缓存重放（镜像
  UploadEventBus.kt），iOS 增强——未 ack 终态落盘（系统后台拉起投递的
  回调在进程重启后同样重放，技术方案 §6.2 持久化原生回调）
- UploadTaskStore.swift：taskId→URLSession taskIdentifier 映射 plist 持久化
  （Application Support；对应 Android WorkManager 自持映射）
- UploadPlugin.swift：URLSession background
  （com.photolive.photo_live.uploads）+ delegate 失败分类（2xx→completed+etag
  / 403→needsRenew / 4xx→HTTP_4XX / 网络→重试上限后 NETWORK，镜像
  UploadPutExecutor/UploadWorker）、原生重试（5 次上限、10s 指数退避）、
  进度 500ms 节流、queryTask/cancelTask/ackEvents、静默取消
- AppDelegate.swift：implicit engine 通道注册 + 启动重建会话 +
  handleEventsForBackgroundURLSession completionHandler 暂存
- Info.plist：UIBackgroundModes 有意不声明（background URLSession 无需；
  计划措辞按技术方案 §6.2 修正，见 README-ios.md）
- project.pbxproj 登记新 Swift 文件；README-ios.md（Xcode 步骤 + G3 验证
  清单：实际文件上传/持久化回调窗口/服务端补偿/重启重联校准/强制退出/
  低电量延迟/100 张批量）

Dart：NativeTransport 选择矩阵并入 iOS（Platform.isAndroid || isIOS，
FORCE_DART_TRANSPORT 覆盖不变）——协议两侧一致，无新通道代码。

flutter analyze 0 issue；flutter test 219 全绿。

Co-Authored-By: Claude Code <noreply@anthropic.com>

apps/mobile/ios/Runner/UploadBridge.swift

@@ -0,0 +1,107 @@
+import Foundation
+
+/// 上传桥接协议常量与 registerTask 参数解析（P3 计划 Task 8）。
+///
+/// 与 Android 侧 `UploadProtocol.kt` 逐字对齐：通道名、方法名、事件名、
+/// 失败码完全一致；Dart 侧 `lib/features/uploads/native_transport.dart`
+/// 的 [UploadBridgeChannels] 为同一协议的单源镜像。
+enum UploadBridge {
+    /// 命令通道（registerTask / cancelTask / queryTask / ackEvents）。
+    static let methodChannel = "photolive/uploads"
+
+    /// 事件流（progress / completed / needsRenew / failed，带单调 seq）。
+    static let eventChannel = "photolive/uploads/events"
+
+    static let methodRegister = "registerTask"
+    static let methodCancel = "cancelTask"
+    static let methodQuery = "queryTask"
+    static let methodAck = "ackEvents"
+
+    static let eventProgress = "progress"
+    static let eventCompleted = "completed"
+    static let eventNeedsRenew = "needsRenew"
+    static let eventFailed = "failed"
+
+    /// 网络不可达/超时（重试上限耗尽后上报）。
+    static let failNetwork = "NETWORK"
+
+    /// 4xx 客户端错误（status 随事件携带；403 单独走 needsRenew）。
+    static let failHttp4xx = "HTTP_4XX"
+
+    /// 5xx 服务端错误（重试上限耗尽后上报）。
+    static let failServerError = "SERVER_ERROR"
+
+    /// 源文件缺失（受管副本被清理，不可恢复）。
+    static let failSourceMissing = "SOURCE_MISSING"
+
+    /// 后台 URLSession 标识（App 重启后系统按此重联会话）。
+    static let sessionIdentifier = "com.photolive.photo_live.uploads"
+}
+
+/// registerTask 参数（Dart → 原生），字段与 Kotlin `UploadRegisterSpec` 一致。
+struct UploadRegisterSpec {
+    let taskId: String
+    let url: String
+    let method: String
+    let headers: [String: String]
+    let filePath: String
+    let uploadId: String?
+    let attempt: Int
+}
+
+/// 参数形状不符（插件层转 bad_args 错误，镜像 Kotlin IllegalArgumentException）。
+struct UploadBridgeBadArgsError: Error {
+    let message: String
+}
+
+/// registerTask 参数解析（纯函数；与 Kotlin `parseRegisterSpec` 同语义）。
+///
+/// 通道解码所得字典为 [String: Any]（Flutter 标准编解码 null → NSNull），
+/// 显式 null 与类型不符同样归为 bad_args。
+func parseRegisterSpec(_ args: [String: Any]) throws -> UploadRegisterSpec {
+    func str(_ key: String) throws -> String {
+        guard let v = args[key] as? String else {
+            throw UploadBridgeBadArgsError(
+                message: args[key] == nil ? "missing '\(key)'" : "'\(key)' must be a String"
+            )
+        }
+        return v
+    }
+    let taskId = try str("taskId")
+    let url = try str("url")
+    let methodRaw = try str("method")
+    let method = methodRaw.isEmpty ? "PUT" : methodRaw
+    let filePath = try str("filePath")
+    var headers: [String: String] = [:]
+    if let raw = args["headers"] as? [String: Any] {
+        for (k, v) in raw {
+            guard let value = v as? String else {
+                throw UploadBridgeBadArgsError(message: "'headers' must be Map<String, String>")
+            }
+            headers[k] = value
+        }
+    } else if args["headers"] != nil {
+        throw UploadBridgeBadArgsError(message: "'headers' must be a Map")
+    }
+    var uploadId: String?
+    var attempt = 0
+    if let metadata = args["metadata"] as? [String: Any] {
+        uploadId = metadata["uploadId"] as? String
+        if let v = metadata["attempt"] as? NSNumber {
+            attempt = v.intValue
+        } else if let v = metadata["attempt"] as? Int {
+            attempt = v
+        } else if metadata["attempt"] != nil {
+            throw UploadBridgeBadArgsError(message: "'metadata.attempt' must be an int")
+        }
+    }
+    return UploadRegisterSpec(
+        taskId: taskId,
+        url: url,
+        method: method,
+        headers: headers,
+        filePath: filePath,
+        uploadId: uploadId,
+        attempt: attempt
+    )
+}