Streaming.cs 789 -> 330. Moved the go-live/record/health operation bodies into a
new MainViewModel.Streaming.Operations.cs partial (459 lines: StartSession,
BeginRecordOnly, BeginGoLive, StartRecordFile, PrepareAndStartLiveAsync,
PollHealthAsync/OnHealthPollTick/ApplyHealthIssue, StopStream,
FinalizeRecordingAsync, UniquePath, Choose/ResetRecordFolder, DefaultRecordFolder,
OnFramePumpFailed/HealthUpdated, ApplyHealth, ResetHealth, BuildEncoderOptions).
Streaming.cs keeps state, quality/resolution tiers, session timer, command
declarations. Pruned dead usings in both files.
Zero behavior change; build 0 warnings; 246 pass, only the 2 known failures.
Docs: ViewModels/index.md Phase 3 tracker updated in same commit.
