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/FirmwareService.swift

@@ -1,7 +1,7 @@
 import Foundation
 
 /// 固件信息
-public struct FirmwareInfo: Codable, Equatable, Sendable {
+public struct FirmwareInfo: Codable, Equatable {
     public let version: String
     public let fileUrl: String
     public let description: String?
@@ -25,20 +25,30 @@ final class FirmwareService {
         self.config = config
     }
 
-    func fetchLatest(identifier: String? = nil, status: String? = nil) async throws -> FirmwareInfo? {
+    func fetchLatest(
+        identifier: String? = nil,
+        status: String? = nil,
+        completion: @escaping (Result<FirmwareInfo?, Error>) -> Void
+    ) {
         let id = (identifier ?? config.firmwareIdentifier).trimmingCharacters(in: .whitespacesAndNewlines)
         let st = status ?? config.firmwareStatus
         guard !id.isEmpty else {
-            throw DuooomiBleError.transferFailed("Invalid firmware identifier")
+            completion(.failure(DuooomiBleError.transferFailed("Invalid firmware identifier")))
+            return
         }
 
-        var comps = URLComponents(url: config.apiHost.appendingPathComponent(latestPath), resolvingAgainstBaseURL: false)!
+        let urlString = "\(config.apiHost)/\(latestPath)"
+        guard var comps = URLComponents(string: urlString) else {
+            completion(.failure(DuooomiBleError.transferFailed("Invalid firmware URL")))
+            return
+        }
         comps.queryItems = [
             URLQueryItem(name: "identifier", value: id),
             URLQueryItem(name: "status", value: st)
         ]
         guard let url = comps.url else {
-            throw DuooomiBleError.transferFailed("Invalid firmware URL")
+            completion(.failure(DuooomiBleError.transferFailed("Invalid firmware URL")))
+            return
         }
 
         var req = URLRequest(url: url)
@@ -46,15 +56,30 @@ final class FirmwareService {
         req.setValue(config.apiKey, forHTTPHeaderField: "x-api-key")
         req.setValue("application/json", forHTTPHeaderField: "accept")
 
-        let (data, resp) = try await URLSession.shared.data(for: req)
-        guard let http = resp as? HTTPURLResponse else {
-            throw DuooomiBleError.transferFailed("Invalid response")
-        }
-        guard (200...299).contains(http.statusCode) else {
-            throw DuooomiBleError.transferFailed("HTTP \(http.statusCode)")
-        }
+        URLSession.shared.dataTask(with: req) { data, resp, error in
+            if let error = error {
+                completion(.failure(error))
+                return
+            }
+            guard let http = resp as? HTTPURLResponse else {
+                completion(.failure(DuooomiBleError.transferFailed("Invalid response")))
+                return
+            }
+            guard (200...299).contains(http.statusCode) else {
+                completion(.failure(DuooomiBleError.transferFailed("HTTP \(http.statusCode)")))
+                return
+            }
+            guard let data = data else {
+                completion(.failure(DuooomiBleError.transferFailed("Empty response")))
+                return
+            }
 
-        let decoded = try JSONDecoder().decode(FirmwareResponse.self, from: data)
-        return decoded.data
+            do {
+                let decoded = try JSONDecoder().decode(FirmwareResponse.self, from: data)
+                completion(.success(decoded.data))
+            } catch {
+                completion(.failure(error))
+            }
+        }.resume()
     }
 }