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/Services/FileTransferService.swift

@@ -9,40 +9,60 @@ final class FileTransferService {
     }
 
     /// 传输文件到设备
-    /// - Parameters:
-    ///   - fileUri: 本地 file:// URL 或远程 https:// URL
-    ///   - commandType: 传输命令类型
-    ///   - onProgress: 进度回调 0.0 ~ 1.0
     func transferFile(
         fileUri: String,
         commandType: CommandType,
-        onProgress: ((Double) -> Void)? = nil
-    ) async throws {
+        onProgress: ((Double) -> Void)? = nil,
+        completion: @escaping (Result<Void, Error>) -> Void
+    ) {
         BleLog.i("Load file: \(fileUri)", "Transfer")
-        let data = try await loadFileData(from: fileUri)
-        BleLog.d("File loaded: size=\(data.count) bytes", "Transfer")
-
-        try await protocolService.send(
-            type: commandType.rawValue,
-            data: data,
-            onProgress: onProgress
-        )
+        loadFileData(from: fileUri) { [weak self] result in
+            switch result {
+            case .failure(let error):
+                completion(.failure(error))
+            case .success(let data):
+                BleLog.d("File loaded: size=\(data.count) bytes", "Transfer")
+                self?.protocolService.send(
+                    type: commandType.rawValue,
+                    data: data,
+                    onProgress: onProgress,
+                    completion: completion
+                )
+            }
+        }
     }
 
-    private func loadFileData(from uri: String) async throws -> Data {
+    private func loadFileData(from uri: String, completion: @escaping (Result<Data, Error>) -> Void) {
         guard let url = URL(string: uri) else {
-            throw DuooomiBleError.transferFailed("Invalid URL: \(uri)")
+            completion(.failure(DuooomiBleError.transferFailed("Invalid URL: \(uri)")))
+            return
         }
 
         if url.isFileURL {
-            return try Data(contentsOf: url)
-        } else {
-            let (data, response) = try await URLSession.shared.data(from: url)
+            do {
+                let data = try Data(contentsOf: url)
+                completion(.success(data))
+            } catch {
+                completion(.failure(DuooomiBleError.transferFailed("Failed to read local file: \(error.localizedDescription)")))
+            }
+            return
+        }
+
+        URLSession.shared.dataTask(with: url) { data, response, error in
+            if let error = error {
+                completion(.failure(DuooomiBleError.transferFailed("Download failed: \(error.localizedDescription)")))
+                return
+            }
             guard let httpResponse = response as? HTTPURLResponse,
                   (200...299).contains(httpResponse.statusCode) else {
-                throw DuooomiBleError.transferFailed("Failed to download file: \(uri)")
+                completion(.failure(DuooomiBleError.transferFailed("Failed to download file: \(uri)")))
+                return
             }
-            return data
-        }
+            guard let data = data, !data.isEmpty else {
+                completion(.failure(DuooomiBleError.transferFailed("Empty download response")))
+                return
+            }
+            completion(.success(data))
+        }.resume()
     }
 }