Add ActiveDevice + DeviceRegistry + HubEnvelopes + IDeviceClient/Channel implementations

backend.tests/FakeDeviceChannel.cs

@@ -1,14 +1,28 @@
 using System.Collections.Concurrent;
 using System.Text.Json;
+using backend.DeviceHub;
 using backend.Tools;
 
 namespace backend.tests;
 
-// Slim version for Tasks 9-15. Task 16 reopens this file to add IDeviceClient + envelope sending.
-public class FakeDeviceChannel : IDeviceChannel
+public class FakeDeviceChannel : IDeviceChannel, IDeviceClient
 {
+    public List<HubEnvelope> SentEnvelopes { get; } = new();
+    public List<byte[]> SentBinary { get; } = new();
     public ConcurrentQueue<JsonElement> PiToolReplies { get; } = new();
 
+    public Task SendEnvelopeAsync<T>(T envelope, CancellationToken ct) where T : HubEnvelope
+    {
+        SentEnvelopes.Add(envelope);
+        return Task.CompletedTask;
+    }
+
+    public Task SendBinaryAsync(ReadOnlyMemory<byte> bytes, CancellationToken ct)
+    {
+        SentBinary.Add(bytes.ToArray());
+        return Task.CompletedTask;
+    }
+
     public Task<JsonElement> CallPiToolAsync(string name, JsonElement args, CancellationToken ct)
     {
         if (PiToolReplies.TryDequeue(out var reply))