Fix first-run setup timing out on the kernel download

On a Mac without the Linux kernel, setup ran `container system start
--enable-kernel-install` with a 120 s limit. That command downloads a
697 MB archive without printing anything, so most connections hit the
limit and the window showed `timedOut([...])`.

- Start the service with --disable-kernel-install (10 min: a first
  start still pulls the 69 MB base filesystem).
- Download the kernel in its own step with progress, then hand the file
  to `container system kernel set --tar ... --digest ...`, which checks
  it against the runtime's digest.
- Opening the setup window no longer starts the download; Set Up does.
- Timeouts and runner errors read as sentences.
- A rebuild deletes the image it replaced, which frees about 2 GB each
  time, and deletes or stops the builder VM unless it was already
  running (it kept about 2.8 GB of memory).
- Setup asks for 10 GB free, measured on a real setup.

Sources/KnotchyCore/Setup/FileDownload.swift

@@ -0,0 +1,100 @@
+import Foundation
+
+/// A data task streamed into a file, so a partial download lives only in its own folder.
+final class FileDownload: NSObject, URLSessionDataDelegate, @unchecked Sendable {
+    // Touched only on the session's serial delegate queue.
+    private var continuation: CheckedContinuation<Void, any Error>?
+    /// A completion that arrived before anyone waited: a task cancelled before it started.
+    private var outcome: Result<Void, any Error>?
+    private var handle: FileHandle?
+    private var written: Int64 = 0
+    private var expected: Int64?
+    private var writeError: (any Error)?
+    private let destination: URL
+    private let progress: @Sendable (_ written: Int64, _ expected: Int64?) -> Void
+
+    private init(destination: URL, progress: @escaping @Sendable (Int64, Int64?) -> Void) {
+        self.destination = destination
+        self.progress = progress
+    }
+
+    /// `expected` is nil when the server doesn't say.
+    static func fetch(_ url: URL, to destination: URL,
+                      progress: @escaping @Sendable (_ written: Int64, _ expected: Int64?) -> Void) async throws {
+        let delegate = FileDownload(destination: destination, progress: progress)
+        let queue = OperationQueue()
+        queue.maxConcurrentOperationCount = 1
+        let session = URLSession(configuration: .ephemeral, delegate: delegate, delegateQueue: queue)
+        defer { session.finishTasksAndInvalidate() }
+        // Created up front: cancelling before it exists would invalidate the session, and creating it then throws.
+        let task = session.dataTask(with: url)
+        try await withTaskCancellationHandler {
+            try await withCheckedThrowingContinuation { (continuation: CheckedContinuation<Void, any Error>) in
+                queue.addOperation {
+                    if let outcome = delegate.outcome {
+                        continuation.resume(with: outcome)
+                    } else {
+                        delegate.continuation = continuation
+                        task.resume()
+                    }
+                }
+            }
+        } onCancel: {
+            task.cancel()
+        }
+    }
+
+    func urlSession(_ session: URLSession, dataTask: URLSessionDataTask, didReceive response: URLResponse,
+                    completionHandler: @escaping @Sendable (URLSession.ResponseDisposition) -> Void) {
+        if let http = response as? HTTPURLResponse, !(200..<300).contains(http.statusCode) {
+            writeError = URLError(.badServerResponse)
+            return completionHandler(.cancel)
+        }
+        do {
+            guard FileManager.default.createFile(atPath: destination.path, contents: nil) else {
+                throw CocoaError(.fileWriteUnknown, userInfo: [NSFilePathErrorKey: destination.path])
+            }
+            handle = try FileHandle(forWritingTo: destination)
+        } catch {
+            writeError = error
+            return completionHandler(.cancel)
+        }
+        if response.expectedContentLength > 0 { expected = response.expectedContentLength }
+        completionHandler(.allow)
+    }
+
+    func urlSession(_ session: URLSession, dataTask: URLSessionDataTask, didReceive data: Data) {
+        guard let handle, writeError == nil else { return }
+        do {
+            try handle.write(contentsOf: data)
+            written += Int64(data.count)
+            progress(written, expected)
+        } catch {
+            writeError = error
+            dataTask.cancel()
+        }
+    }
+
+    func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: (any Error)?) {
+        try? handle?.close()
+        handle = nil
+        // Ours comes first: cancelling after it adds a "cancelled" error.
+        let result: Result<Void, any Error> = if let failure = writeError ?? error { .failure(failure) } else { .success(()) }
+        if let waiting = continuation {
+            continuation = nil
+            waiting.resume(with: result)
+        } else {
+            outcome = result
+        }
+    }
+}
+
+extension FileDownload {
+    static func removeFolders(prefix: String) {
+        let temporary = URL(fileURLWithPath: NSTemporaryDirectory())
+        let names = (try? FileManager.default.contentsOfDirectory(atPath: temporary.path)) ?? []
+        for name in names where name.hasPrefix(prefix) {
+            try? FileManager.default.removeItem(at: temporary.appendingPathComponent(name))
+        }
+    }
+}