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.

Models/YouTube.cs

@@ -31,4 +31,9 @@ public class ChatMessage
     public string? SuperChatCurrency { get; set; }
     public bool IsMember { get; set; }
     public string? MembershipLevel { get; set; }
+
+    /// <summary>TASK 41: the Test Stream dock injects synthetic member / SuperChat /
+    /// subscriber events locally (YouTube's insert API only supports text), so the
+    /// buffer can label them honestly — simulated events never reach YouTube.</summary>
+    public bool IsSimulated { get; set; }
 }