feat(models): 模型自动下载(我的·模型管理) + 断点续传 + 旁路导入

实现 spec(2026-05-29-model-download-design)的模型分发功能:
- ModelManifest: 硬编码功能文件清单 + base URL https://file.myv0.com/
- FileDownloader: URLSessionDataDelegate 分块写盘,HTTP Range 断点续传 + 大小校验
  (根因修复:URL.resourceValues 会缓存文件大小,续传时先读 offset 再读 finalSize
   会拿到下载前的陈旧值导致校验误判;改用 FileManager.attributesOfItem)
- ModelDownloadService: @MainActor @Observable 编排逐文件下载,聚合进度/速度,
  支持下载全部/暂停/重试,以及旁路文件导入
- ModelStore: 新增 fileURL/localBytes/isComplete(可注入清单)/importModel(补 VL)
- ModelManagementView: 分模型卡片(状态/进度/速度) + 下载全部/暂停
  + NWPathMonitor 蜂窝提示 + 从文件导入(离线兜底)
- MeView: 模型管理卡改 NavigationLink + 动态状态(已就绪/下载中/N就绪)

测试(Swift Testing): Manifest 清单/字节数、Store 路径/校验/导入、
DownloadState、FileDownloader(URLProtocol mock:下载/Range续传/大小校验)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

康康/AI/FileDownloader.swift

@@ -0,0 +1,158 @@
+import Foundation
+
+enum DownloadError: Error, LocalizedError {
+    case badStatus(Int)
+    case sizeMismatch(expected: Int, got: Int)
+
+    var errorDescription: String? {
+        switch self {
+        case .badStatus(let code):
+            return "下载失败(HTTP \(code))"
+        case .sizeMismatch(let expected, let got):
+            return "文件大小校验失败(预期 \(expected),实际 \(got))"
+        }
+    }
+}
+
+/// 下载单个文件,支持 HTTP Range 断点续传 + 完成后大小校验。
+/// 用 `URLSessionDataDelegate` 把响应体分块写入 `.part`,完成后原子改名为成品。
+///
+/// 注意:文件大小一律用 `FileManager.attributesOfItem` 读取,**不用**
+/// `URL.resourceValues(.fileSizeKey)` —— 后者会把结果缓存在 URL 实例上,
+/// 续传时先读 offset 再读 finalSize 会拿到下载前的陈旧大小,导致误判校验失败。
+///
+/// 一个实例一次处理一个文件(串行)。共享状态用锁保证可见性。
+final class FileDownloader: NSObject, URLSessionDataDelegate, @unchecked Sendable {
+    private let configuration: URLSessionConfiguration
+
+    private let lock = NSLock()
+    private var handle: FileHandle?
+    private var written: Int = 0
+    private var onProgress: ((Int) -> Void)?
+    private var responseError: Error?
+    private var continuation: CheckedContinuation<Void, Error>?
+
+    init(configuration: URLSessionConfiguration = .default) {
+        self.configuration = configuration
+        super.init()
+    }
+
+    /// 不走 URL 资源值缓存的文件大小读取。
+    static func fileSize(at url: URL) -> Int {
+        guard let attrs = try? FileManager.default.attributesOfItem(atPath: url.path),
+              let size = attrs[.size] as? Int else { return 0 }
+        return size
+    }
+
+    /// 从 `url` 下载到 `destination`。若存在 `destination.part` 则发 Range 请求续传;
+    /// 完成后校验总大小 == `expectedBytes`,通过则原子改名为 `destination`。
+    nonisolated func download(
+        from url: URL,
+        to destination: URL,
+        expectedBytes: Int,
+        onProgress: (@Sendable (Int) -> Void)? = nil
+    ) async throws {
+        let fm = FileManager.default
+        let part = destination.appendingPathExtension("part")
+
+        // 成品已存在且大小正确 → 跳过
+        if Self.fileSize(at: destination) == expectedBytes,
+           fm.fileExists(atPath: destination.path) {
+            return
+        }
+
+        try fm.createDirectory(
+            at: destination.deletingLastPathComponent(), withIntermediateDirectories: true)
+
+        var offset = 0
+        if fm.fileExists(atPath: part.path) {
+            offset = Self.fileSize(at: part)
+        } else {
+            fm.createFile(atPath: part.path, contents: nil)
+        }
+
+        let fileHandle = try FileHandle(forWritingTo: part)
+        try fileHandle.seekToEnd()
+
+        lock.lock()
+        self.handle = fileHandle
+        self.written = offset
+        self.onProgress = onProgress
+        self.responseError = nil
+        lock.unlock()
+
+        var request = URLRequest(url: url)
+        if offset > 0 {
+            request.setValue("bytes=\(offset)-", forHTTPHeaderField: "Range")
+        }
+
+        let session = URLSession(configuration: configuration, delegate: self, delegateQueue: nil)
+        defer { session.finishTasksAndInvalidate() }
+
+        // 句柄在 didCompleteWithError 内关闭(同一 delegate 队列,串行于 didReceive)。
+        try await withCheckedThrowingContinuation { (cont: CheckedContinuation<Void, Error>) in
+            lock.lock()
+            self.continuation = cont
+            lock.unlock()
+            session.dataTask(with: request).resume()
+        }
+
+        let finalSize = Self.fileSize(at: part)
+        guard finalSize == expectedBytes else {
+            try? fm.removeItem(at: part)
+            throw DownloadError.sizeMismatch(expected: expectedBytes, got: finalSize)
+        }
+
+        if fm.fileExists(atPath: destination.path) {
+            try fm.removeItem(at: destination)
+        }
+        try fm.moveItem(at: part, to: destination)
+    }
+
+    // MARK: - URLSessionDataDelegate (全部在串行 delegate 队列执行)
+
+    nonisolated func urlSession(
+        _ session: URLSession, dataTask: URLSessionDataTask,
+        didReceive response: URLResponse,
+        completionHandler: @escaping (URLSession.ResponseDisposition) -> Void
+    ) {
+        if let http = response as? HTTPURLResponse, http.statusCode >= 400 {
+            lock.lock(); responseError = DownloadError.badStatus(http.statusCode); lock.unlock()
+            completionHandler(.cancel)
+        } else {
+            completionHandler(.allow)
+        }
+    }
+
+    nonisolated func urlSession(
+        _ session: URLSession, dataTask: URLSessionDataTask, didReceive data: Data
+    ) {
+        lock.lock()
+        try? handle?.write(contentsOf: data)
+        written += data.count
+        let progress = written
+        let callback = onProgress
+        lock.unlock()
+        callback?(progress)
+    }
+
+    nonisolated func urlSession(
+        _ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?
+    ) {
+        lock.lock()
+        try? handle?.close()
+        handle = nil
+        let cont = continuation
+        continuation = nil
+        let respErr = responseError
+        lock.unlock()
+
+        if let respErr {
+            cont?.resume(throwing: respErr)
+        } else if let error {
+            cont?.resume(throwing: error)
+        } else {
+            cont?.resume()
+        }
+    }
+}