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>

Package.swift

@@ -1,11 +1,11 @@
-// swift-tools-version: 5.9
+// swift-tools-version: 5.0
 
 import PackageDescription
 
 let package = Package(
     name: "DuooomiBleSDK",
     platforms: [
-        .iOS(.v16)
+        .iOS(.v12)
     ],
     products: [
         .library(