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>

Sources/DuooomiBleSDK/DuooomiBleSDKDelegate.swift

@@ -0,0 +1,34 @@
+import Foundation
+
+/// SDK 状态变化回调协议，替代 Combine @Published 观察。
+/// 所有方法在主线程调用。
+public protocol DuooomiBleSDKDelegate: AnyObject {
+    /// 蓝牙连接状态变化
+    func sdk(_ sdk: DuooomiBleSDK, didChangeState state: ConnectionState)
+    /// 扫描到的设备列表更新
+    func sdk(_ sdk: DuooomiBleSDK, didUpdateDevices devices: [DiscoveredDevice])
+    /// 已连接设备变化
+    func sdk(_ sdk: DuooomiBleSDK, didConnectDevice device: DiscoveredDevice?)
+    /// 设备信息更新
+    func sdk(_ sdk: DuooomiBleSDK, didUpdateDeviceInfo info: DeviceInfo?)
+    /// 固件版本更新
+    func sdk(_ sdk: DuooomiBleSDK, didUpdateVersion version: String)
+    /// 激活状态变化
+    func sdk(_ sdk: DuooomiBleSDK, didChangeActivation isActivated: Bool)
+    /// 传输进度更新 (0-100)
+    func sdk(_ sdk: DuooomiBleSDK, didUpdateProgress progress: Int)
+    /// 错误发生
+    func sdk(_ sdk: DuooomiBleSDK, didEncounterError error: String?)
+}
+
+/// 所有 delegate 方法提供默认空实现，调用方只需实现关心的方法。
+public extension DuooomiBleSDKDelegate {
+    func sdk(_ sdk: DuooomiBleSDK, didChangeState state: ConnectionState) {}
+    func sdk(_ sdk: DuooomiBleSDK, didUpdateDevices devices: [DiscoveredDevice]) {}
+    func sdk(_ sdk: DuooomiBleSDK, didConnectDevice device: DiscoveredDevice?) {}
+    func sdk(_ sdk: DuooomiBleSDK, didUpdateDeviceInfo info: DeviceInfo?) {}
+    func sdk(_ sdk: DuooomiBleSDK, didUpdateVersion version: String) {}
+    func sdk(_ sdk: DuooomiBleSDK, didChangeActivation isActivated: Bool) {}
+    func sdk(_ sdk: DuooomiBleSDK, didUpdateProgress progress: Int) {}
+    func sdk(_ sdk: DuooomiBleSDK, didEncounterError error: String?) {}
+}