refactor: concentrate Cast ownership and preserve intent on disconnect

Sources/App/CastSession.swift

@@ -238,6 +238,7 @@ enum CastWire {
 final class CastSession {
     var onStatus: ((CastStatus) -> Void)?
     var onDisconnect: ((String?) -> Void)?
+    // Recoverable command errors; terminal channel failures belong to onDisconnect.
     var onError: ((String) -> Void)?
     var localHost: String? {
         guard let endpoint = connection?.currentPath?.localEndpoint,
@@ -499,7 +500,7 @@ final class CastSession {
                         destination: receiver ? "receiver-0" : transport,
                         operation: (command["type"] as? String ?? "command").lowercased(),
                         generation: mediaGeneration, id: id) { [weak self] _, error in
-                guard let self else { return }
+                guard let self, self.connection != nil else { return }
                 if self.pendingVolume?.request == id { self.pendingVolume = nil; self.publishSeekState() }
                 if case .cancelled? = error { return }
                 if self.pendingSeek == id { self.pendingSeek = nil; self.publishSeekState() }
@@ -760,7 +761,7 @@ final class CastSession {
                             namespace: CastWire.media, destination: transport, operation: "media status",
                             generation: self.mediaGeneration) { [weak self] _, error in
                                 if case .cancelled? = error { return }
-                                if let error { self?.onError?(error.localizedDescription) }
+                                if let self, self.connection != nil, let error { self.onError?(error.localizedDescription) }
                             }
                     }
                 } catch { self.fail(error as? CastError ?? .disconnected); return }