refactor: extract recording-output concern into MainViewModel.Recording.cs — Commit F

The original premise (extract FFmpegEncoder / StreamHealthMonitor / FramePump
classes) was a no-op — those already exist as Services/Encoder/FfmpegEncoder.cs
and Services/Encoder/FramePump.cs. The honest functional seam was the local
recording-output concern, which was interleaved with live-stream orchestration:

- New MainViewModel.Recording.cs (121): StartRecordFile, FinalizeRecordingAsync,
  UniquePath, ChooseRecordFolder, ResetRecordFolder, DefaultRecordFolder + the
  recording fields (_recordFolder, RecordFolderDisplay, _activeRecordPath,
  _recordStartTime, _recordLength).
- MainViewModel.Streaming.Operations.cs 475 -> 377 (keeps live session lifecycle,
  health, visuals).
- MainViewModel.Streaming.cs 328 -> 323 (keeps pills/state/properties intact).

Same partial class — all MVVM bound-property glue untouched, no behavior change.
Build 0 warnings; 246 pass, only the 2 known failures.
Docs: ViewModels/index.md tracker updated in same commit.

ViewModels/MainViewModel.Streaming.cs

@@ -38,14 +38,9 @@ public partial class MainViewModel : ViewModelBase
     private bool _recordPillOn;
     private bool _onAirPillOn;
     private bool _isRecording;
-    private string? _recordFolder;
 
     /// <summary>Resolved record folder — the configured choice, or Downloads (0th case).</summary>
-    public string RecordFolderDisplay => _recordFolder ?? DefaultRecordFolder();
 
-    private string? _activeRecordPath;
-    private DateTime _recordStartTime;
-    private TimeSpan _recordLength;
 
     private readonly FramePump _framePump;