fix: webcam identity->device key for output frames; top bar always has a Start; light-pill grouping

Take two (2026-09-01) three confirmed defects, all from creator feedback + the
new resolver logging:

1. NO WEBCAM IN OUTPUT: WebcamSceneConfig.WebcamId carries the identity GUID;
   CameraManager keys sessions by DEVICE id — GetLatestFrame(guid) returned null
   forever, the compositor silently dropped the layer while the preview (bitmap
   path) looked fine. The resolver logging added in 8dcaee0 caught it red-handed.
   DeviceKeyForWebcam maps identity->device through the _webcam singleton (identity
   mismatch / unknown ids pass through). Test: WebcamOutputKeyTests.

2. START BUTTON VANISHED AFTER STOP: my pills-clear ruling met the old
   ShowPrimaryStartButton gate (needed IsConnected or a lit REC pill) — signed-out,
   pills-off = blank top bar, no session reachable. Start is now ALWAYS the idle
   face; unarmed Start records (local needs no account — no dead-end no-op); the
   Sign In button folds into Start's context menu ('Sign in to YouTube', shown while
   disconnected) with a dedicated SignInCommand. SessionTeardownTests extended:
   stopped session must leave a reachable Start.

3. TOP BAR ORDER (creator spec): [sign light] REC [pill]  [sign light] ON-AIR [pill]
   — each reality lamp now sits in front of its own intent switch (was: two pills,
   then two orphaned dots). Sign labels keep the shared StatusSignText style.

Tests: WebcamOutputKey, SessionTeardown, FramePump, WebcamMenuGate, GlobalHotkey,
BroadcastPullOut — 14/14 across the six classes, clean build 0 warnings.

ViewModels/MainViewModel.cs

@@ -211,6 +211,7 @@ public partial class MainViewModel : ViewModelBase
         OpenPremiumCommand = new RelayCommand(_ => OpenUrl(PremiumUrl), _ => IsPremiumAvailable);
         OpenLicensesCommand = new RelayCommand(_ => ShowLicenses());
         StartStreamCommand = new RelayCommand(_ => StartSession());
+        SignInCommand = new RelayCommand(_ => _ = SignInAsync(), _ => !IsConnected);
         EndStreamCommand = new RelayCommand(_ => StopStream(), _ => IsLive || IsRecording);
         ChooseRecordFolderCommand = new RelayCommand(_ => ChooseRecordFolder());
         ResetRecordFolderCommand = new RelayCommand(_ => ResetRecordFolder());
@@ -478,11 +479,24 @@ public partial class MainViewModel : ViewModelBase
 
     private DateTime _lastWebcamNullLog = DateTime.MinValue;
 
+    /// <summary>Maps a config's identity GUID to the CameraManager session key (the
+    /// DEVICE id). internal = test seam. Take-2 fix: looking up the identity GUID
+    /// against a device-keyed dictionary returned null forever.</summary>
+    internal string DeviceKeyForWebcam(string webcamId) =>
+        _webcam is { } w && w.Id == webcamId && !string.IsNullOrWhiteSpace(w.DeviceId)
+            ? w.DeviceId
+            : webcamId;
+
     private VideoFrame? ResolveOutputFrameCore(SceneElement element)
     {
         return element switch
         {
-            WebcamSceneConfig webcam => _cameraManager.GetLatestFrame(webcam.WebcamId),
+            // CameraManager sessions are keyed by DEVICE id (AcquireAsync(_webcam.DeviceId));
+            // configs carry the identity GUID (WebcamSceneConfig.WebcamId = _webcam.Id).
+            // Looking up by the GUID returned null forever — the webcam was structurally
+            // invisible on the output while the preview (bitmap path) looked fine
+            // (caught by the resolver logging, 2026-09-01 take two).
+            WebcamSceneConfig webcam => _cameraManager.GetLatestFrame(DeviceKeyForWebcam(webcam.WebcamId)),
             Source { IsLiveCapture: true, CaptureKey: not null } live => _screenCaptureManager.GetLatestFrame(live.CaptureKey),
             Source { Type: SourceType.ChatBox } chatBox => RenderChatBox(chatBox),
             Source { Type: SourceType.WebSource } web => _webView2Manager?.GetLatestFrame(web.Id),