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>

DuooomiBleSDK.podspec

@@ -8,10 +8,10 @@ Pod::Spec.new do |s|
   s.author       = { 'comi-logic' => 'dev@bowong.cc' }
   s.source       = { :git => 'https://gitea.bowong.cc/comi-logic/duooomi-ios-sdk.git', :tag => s.version.to_s }
 
-  s.ios.deployment_target = '16.0'
+  s.ios.deployment_target = '12.2'
   s.swift_version = '5.0'
 
   s.source_files = 'Sources/DuooomiBleSDK/**/*.swift'
 
-  s.frameworks = 'CoreBluetooth', 'Foundation', 'Combine'
+  s.frameworks = 'CoreBluetooth', 'Foundation'
 end