feat(stream): TASK 41 Test Stream mode — private test broadcast + TEST drawer

Test button next to Start runs the real private-only go-live pipeline as a
session variant (IsTestStream): BeginTestStream -> extracted
StartStreamingSession(alsoRecord:false) shared with the dialog path — no Go
Live dialog, no 'last live' stamp, no recording. Gold top bar + TEST badge +
'End Test' button face; TEST drawer (third right-rail pull-out, three-way
exclusivity) auto-opens once the liveChatId resolves.

Chat tooling: Mock Chat Input sends a REAL liveChat/messages.insert
(YouTubeStreamService.InsertChatMessageAsync) that round-trips the real ~2s
poll and renders through the live overlay; simulated Subscriber/New
Member/Super Chat events inject through the poller's MessageReceived seam
(YouTubeChatService.InjectSimulatedMessage, ChatMessage.IsSimulated) — the
insert API only creates textMessageEvent, so non-text events are local-only
by design (ref: developers.google.com/youtube/v3/live/docs/liveChatMessages/insert).
Scopes youtube + youtube.force-ssl already cover insert; no re-consent.

Good Dog: ytLive.Tests/TestStreamTests.cs — coordinator protocol with fake
HTTP + real-window drawer exclusivity/gate. 0 warnings; 314/314 pass (clean
run; intermittent host abort is the pre-existing WinRT-webcam flake).

Docs ride in the same change: TASKS.md row 41, TASKS/task-41-test-stream-mode.md,
ai.md (YouTube Live API constraints), HANDOFF rewrite.

ViewModels/MainViewModel.Streaming.cs

@@ -41,6 +41,13 @@ public partial class MainViewModel : ViewModelBase
     private bool _onAirPillOn;
     private bool _isRecording;
 
+    /// <summary>TASK 41: the current session is a private TEST broadcast — the real
+    /// go-live pipeline runs (broadcast create, reusable stream, RTMP push, chat
+    /// poll), but the visuals say TEST (top-bar gold + badge), no "last live" stamp
+    /// is written, and the Test drawer is open. One flag flips the whole face of the
+    /// session; cleared on stop / rollback.</summary>
+    private bool _isTestStream;
+
     /// <summary>Resolved record folder — the configured choice, or Downloads (0th case).</summary>
 
 
@@ -64,6 +71,7 @@ public partial class MainViewModel : ViewModelBase
                 OnPropertyChanged(nameof(ShowPrimaryStartButton));
                 OnPropertyChanged(nameof(ShowEndStreamButton));
                 OnPropertyChanged(nameof(EndButtonText));
+                OnPropertyChanged(nameof(ShowTestButton));
                 OnPropertyChanged(nameof(AccountStatusLightToolTip));
                 OnPropertyChanged(nameof(ShowChatInactiveMessage));
                 OnPropertyChanged(nameof(ShowPreviewPlaceholder));
@@ -126,6 +134,7 @@ public partial class MainViewModel : ViewModelBase
             OnPropertyChanged(nameof(ShowEndStreamButton));
             OnPropertyChanged(nameof(ShowPrimaryStartButton));
             OnPropertyChanged(nameof(EndButtonText));
+            OnPropertyChanged(nameof(ShowTestButton));
             UpdateSessionTimer();
         }
     }
@@ -154,8 +163,35 @@ public partial class MainViewModel : ViewModelBase
     public bool ShowEndStreamButton => IsLive || IsRecording;
 
     /// <summary>The one button whose label changes: while recording it reads
-    /// "Stop Recording", while live "End Stream".</summary>
-    public string EndButtonText => IsRecording && !IsLive ? "Stop Recording" : "End Stream";
+    /// "Stop Recording", while live "End Stream" — and while a test broadcast runs,
+    /// "End Test" so the creator knows exactly what they're stopping.</summary>
+    public string EndButtonText =>
+        IsRecording && !IsLive ? "Stop Recording"
+        : IsTestStream ? "End Test"
+        : "End Stream";
+
+    /// <summary>TASK 41 — the Test button (top bar, left of Start): a one-click
+    /// private test broadcast. Only meaningful signed-in and idle — it disappears
+    /// the moment a session (live OR recording) starts.</summary>
+    public bool ShowTestButton => IsOffline && !IsRecording && IsConnected;
+
+    public bool CanStartTest => ShowTestButton;
+
+    /// <summary>TASK 41 — the current session is a test broadcast (drives the TEST
+    /// badge + gold top bar + "End Test" face). Internal setter is a test seam like
+    /// IsRecording: <see cref="BeginTestStream"/> flips it before <see cref="StreamStatus"/>
+    /// goes Streaming so the visuals initialize amber, and the stop/rollback path
+    /// clears it.</summary>
+    public bool IsTestStream
+    {
+        get => _isTestStream;
+        internal set
+        {
+            if (!SetProperty(ref _isTestStream, value)) return;
+            OnPropertyChanged(nameof(EndButtonText));
+            UpdateLiveVisuals();
+        }
+    }
 
     public StreamHealth CurrentHealth
     {
@@ -330,6 +366,7 @@ public partial class MainViewModel : ViewModelBase
     }
 
     public ICommand StartStreamCommand { get; }
+    public ICommand StartTestCommand { get; }
     public ICommand EndStreamCommand { get; }
     public ICommand SignInCommand { get; }
     public ICommand ChooseRecordFolderCommand { get; }