fab2e09903
Background.cs 567 -> 418. Moved the static scene-model helpers (LoadBackgroundImage, CreatePlaceholderSnapshot, EnsureBackground, CreateBackground, NormalizeBackgrounds, AllBackgrounds, ResolveAutoCaptureKey, MonitorKeyPrefix) into a new MainViewModel.Background.Model.cs partial. Background.cs keeps capture/live state + operations. Zero behavior change; build 0 warnings; 246 pass, only the 2 known failures. Docs: ViewModels/index.md Phase 3 tracker updated in same commit.