feat(webcam): resource lifecycle startup slice — poll-on-start, single-cam lock, persistent Layers alert

The creator couldn't add a webcam to Live (grayed app-wide) and nothing in the
app explained why. Ground truth from the live DB: one Webcam identity AND one
WebcamSceneConfig in the Chat scene — so the gray was the single-identity rule
working, but the reason was unobservable. This slice makes the webcam a
resource the app validates and locks, mirroring how OBS reserves its devices.

At startup we enumerate the OS once (ValidateWebcamResourceStartupAsync, fired
after LoadLayout, stored as WebcamStartupValidationTask for tests to await):
- 0 webcams -> app runs on, layer inactive, no alarm
- exactly 1 -> attempt CameraManager.AcquireAsync as an app-wide lock; on
  failure show a persistent red alert at the bottom of the Layers panel
  (WebcamLockAlert + Retry) that re-polls every 5s and clears itself the
  moment the camera locks, or on any first real frame
- >=2 -> deliberately no auto-lock; camera selection belongs to the App
  Settings dialog (gear) — next slice

CameraManager.IsRunning(deviceId) tells the pass a session already exists
(started OR still starting) so loaded identity configs count as the lock and
the pass never double-acquires. Test seams mirror LayoutPathOverride:
CameraEnumeratorOverride / CameraFrameSourceFactoryOverride so the startup
probe never touches real hardware under test.

Good Dog test: WebcamStartupResourceTests x3 — single-cam locked + app runs on,
zero-cams no-alarm/no-lock, lock-fails -> red alert -> Retry -> clears. Full
suite 304/304, build 0 warnings, scope-check passed.

Docs in-commit: TASKS Open items + ai.md Webcam section + HANDOFF rewrite.
Also corrects the record: 'NVIDIA Broadcast opens the webcam exclusively' was a
suspect-list claim (CameraConflictProbe reads process names only, no handles)
— not restated as fact.

ViewModels/MainViewModel.cs

@@ -215,6 +215,7 @@ public partial class MainViewModel : ViewModelBase
         EndStreamCommand = new RelayCommand(_ => StopStream(), _ => IsLive || IsRecording);
         ChooseRecordFolderCommand = new RelayCommand(_ => ChooseRecordFolder());
         ResetRecordFolderCommand = new RelayCommand(_ => ResetRecordFolder());
+        RetryWebcamLockCommand = new RelayCommand(_ => _ = ValidateWebcamResourceStartupAsync());
         ActivateLicenseCommand = new RelayCommand(_ => _ = ActivateLicenseAsync(), _ => !IsLicenseValidating);
         OpenLicenseEntryCommand = new RelayCommand(_ => IsLicenseEntryOpen = !IsLicenseEntryOpen);
         OpenHotkeyConfigCommand = new RelayCommand(_ => OpenHotkeyConfig());
@@ -240,10 +241,10 @@ public partial class MainViewModel : ViewModelBase
         MicSourceName = _layoutStore.LoadMicSourceName();
         AudioSyncOffsetMs = _layoutStore.LoadAudioSyncOffsetMs();
 
-        _cameraEnumerator = new MediaCaptureCameraEnumerator();
+        _cameraEnumerator = CameraEnumeratorOverride ?? new MediaCaptureCameraEnumerator();
         _cameraManager = new CameraManager(
             _cameraEnumerator,
-            id => new MediaCaptureFrameSource(id),
+            CameraFrameSourceFactoryOverride ?? (id => new MediaCaptureFrameSource(id)),
             System.Windows.Application.Current?.Dispatcher);
         _cameraManager.PreviewBitmapChanged += OnCameraPreviewBitmapChanged;
         _cameraManager.CameraFailed += OnCameraFailed;
@@ -314,6 +315,11 @@ public partial class MainViewModel : ViewModelBase
         _framePump.HealthUpdated += OnFramePumpHealthUpdated;
 
         LoadLayout();
+        // Resource validation runs as soon as the layout is up: enumerate the
+        // OS once, and if exactly one webcam exists, lock it for the session
+        // (the saved identity's scene configs may already hold it — refcounted).
+        // Failures surface as a persistent red alert in the Layers panel.
+        WebcamStartupValidationTask = ValidateWebcamResourceStartupAsync();
         // The cached reusable stream gives the pump its RTMP URL the moment go-live
         // starts (the pump reads the URL once at startup — no waiting on YouTube).
         _reusableStream = _layoutStore.LoadReusableStream();
@@ -333,6 +339,12 @@ public partial class MainViewModel : ViewModelBase
     // this to a temp path before constructing MainWindow and reset it after.
     internal static string? LayoutPathOverride { get; set; }
 
+    // Test seams (same pattern): startup resource validation must never touch a
+    // real camera while under test. Tests inject a fake enumerator / frame-source
+    // factory before constructing MainWindow and reset them after.
+    internal static ICameraEnumerator? CameraEnumeratorOverride { get; set; }
+    internal static Func<string, ICameraFrameSource>? CameraFrameSourceFactoryOverride { get; set; }
+
     private void LoadLayout()
     {
         _isLoading = true;