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/Models/DeleteFileResponse.swift

@@ -1,6 +1,6 @@
 import Foundation
 
-public struct DeleteFileResponse: Codable, Sendable, Equatable {
+public struct DeleteFileResponse: Codable, Equatable {
     public let type: Int
     /// 0 = 成功, 1 = 失败, 2 = 文件不存在
     public let success: Int