MainViewModel is now a partial class — scaffold for the per-area split (Commit 0)

Zero behavior change: \`public class\` -> \`public partial class\`. Each functional area
(Scenes, Background, Webcam, Audio, Trax, Socials, Streaming, Chat, Overlays,
Account, License) will move to its own file next, one commit per partial — each
a roll-back point. Docs updated in the same commit: ViewModels/index.md (split
map), ai.md (architecture table), HANDOFF.md (state). verify.sh: build 0 warnings,
only the 2 known pre-existing test failures.

ViewModels/MainViewModel.cs

@@ -21,7 +21,7 @@ using ytLive.Services.Encoder;
 
 namespace ytLive.ViewModels;
 
-public class MainViewModel : ViewModelBase
+public partial class MainViewModel : ViewModelBase
 {
     private readonly YouTubeAuthService _youtubeAuth;
     private readonly YouTubeStreamService _youtubeStream;