feat: 重写 SDK 支持 iOS 12.2，移除 async/await 和 Combine

- 所有公开 API 改为 completion handler (Result<T, Error>)
- 状态通知改为 DuooomiBleSDKDelegate 协议
- 移除 Sendable、@MainActor、AsyncStream、CheckedContinuation
- 网络请求改为 URLSession.dataTask 回调
- BLE 通信改为 delegate + 闭包模式
- deployment target 降至 iOS 12.2

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

demo/Sources/DemoApp.swift

@@ -2,19 +2,69 @@ import SwiftUI
 import DuooomiBleSDK
 
 /// Demo 入口：初始化 SDK 并注入到视图树。
-/// 只需传 apiKey，其余配置（apiHost/cdnHost/firmwareIdentifier/firmwareStatus）使用默认值。
 @main
 struct DemoApp: App {
-    @StateObject private var sdk = DuooomiBleSDK(config: .init(
-        apiKey: ""
-    ))
+    @StateObject private var viewModel = DemoViewModel()
 
     var body: some Scene {
         WindowGroup {
             NavigationView {
-                WrapperTestView()
+                WrapperTestView(viewModel: viewModel)
             }
-            .environmentObject(sdk)
         }
     }
 }
+
+/// ViewModel 包装 SDK callback API，通过 DuooomiBleSDKDelegate 驱动 UI 更新。
+class DemoViewModel: NSObject, ObservableObject, DuooomiBleSDKDelegate {
+    let sdk: DuooomiBleSDK
+
+    @Published var btState: ConnectionState = .idle
+    @Published var connectedDevice: DiscoveredDevice? = nil
+    @Published var discoveredDevices: [DiscoveredDevice] = []
+    @Published var deviceInfo: DeviceInfo? = nil
+    @Published var version: String = ""
+    @Published var isActivated: Bool = false
+    @Published var transferProgress: Int = 0
+    @Published var error: String? = nil
+
+    override init() {
+        sdk = DuooomiBleSDK(config: .init(apiKey: ""))
+        super.init()
+        sdk.delegate = self
+    }
+
+    // MARK: - DuooomiBleSDKDelegate
+
+    func sdk(_ sdk: DuooomiBleSDK, didChangeState state: ConnectionState) {
+        btState = state
+    }
+
+    func sdk(_ sdk: DuooomiBleSDK, didUpdateDevices devices: [DiscoveredDevice]) {
+        discoveredDevices = devices
+    }
+
+    func sdk(_ sdk: DuooomiBleSDK, didConnectDevice device: DiscoveredDevice?) {
+        connectedDevice = device
+    }
+
+    func sdk(_ sdk: DuooomiBleSDK, didUpdateDeviceInfo info: DeviceInfo?) {
+        deviceInfo = info
+    }
+
+    func sdk(_ sdk: DuooomiBleSDK, didUpdateVersion version: String) {
+        self.version = version
+    }
+
+    func sdk(_ sdk: DuooomiBleSDK, didChangeActivation isActivated: Bool) {
+        self.isActivated = isActivated
+    }
+
+    func sdk(_ sdk: DuooomiBleSDK, didUpdateProgress progress: Int) {
+        transferProgress = progress
+    }
+
+    func sdk(_ sdk: DuooomiBleSDK, didEncounterError error: String?) {
+        self.error = error
+    }
+}