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/BindingResponse.swift

@@ -1,6 +1,6 @@
 import Foundation
 
-public struct BindingResponse: Codable, Sendable, Equatable {
+public struct BindingResponse: Codable, Equatable {
     public let type: Int
     /// 设备 SN 码 (14 位)
     public let sn: String

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

Sources/DuooomiBleSDK/Models/DeviceInfo.swift

@@ -1,6 +1,6 @@
 import Foundation
 
-public struct DeviceInfo: Sendable, Equatable {
+public struct DeviceInfo: Equatable {
     public let allspace: UInt64
     public let freespace: UInt64
     public let name: String

Sources/DuooomiBleSDK/Models/PrepareTransferResponse.swift

@@ -1,6 +1,6 @@
 import Foundation
 
-public struct PrepareTransferResponse: Codable, Sendable, Equatable {
+public struct PrepareTransferResponse: Codable, Equatable {
     public let type: Int
     public let key: String
     /// "ready" | "no_space" | "duplicated"

Sources/DuooomiBleSDK/Models/UnbindResponse.swift

@@ -1,6 +1,6 @@
 import Foundation
 
-public struct UnbindResponse: Codable, Sendable, Equatable {
+public struct UnbindResponse: Codable, Equatable {
     /// 1 = 成功, 0 = 失败
     public let success: Int
 }

Sources/DuooomiBleSDK/Models/VersionInfo.swift

@@ -1,6 +1,6 @@
 import Foundation
 
-public struct VersionInfo: Sendable, Equatable {
+public struct VersionInfo: Equatable {
     public let version: String
     /// 命令类型标识，设备可能返回 Int (如 7) 或 String (如 "0x07")
     public let type: String