feat(alerts): TASK 43 — native events & alerts (six unique animations) + chat parity

Replace the external StreamElements feed with NATIVE YouTube events per the creator's
2026-09-23 question.

Chat parity half:
- YouTubeChatService decodes ALL SIX liveChat/messages event types into
  ChatMessage.Kind (superChat/superSticker/newSponsor/membershipGifting/
  giftMembershipReceived/memberMilestoneChat) — the four formerly-empty overlay
  rows are real now — and re-arms its one-shot poll on the server's
  pollingIntervalMillis (streamList connection semantics; clamp 1000-6000ms,
  maxResults=2000). ParsePage internal static seam + ChatPage record for
  deterministic tests; optional HttpClient ctor seam kept; InjectSimulatedMessage
  (TASK 41) preserved.

Alerts half (creator rulings: one celebration zone, six UNIQUE animations, no
menus/polls, sub mention = chat row only, NO viewer count):
- New SourceType.AlertBox 'Stream Alerts' (one per layout, CanAddAlerts gate
  mirroring chat; idle = transparent).
- AlertRenderer: six distinct branded animations (SuperChat slide-up/shine/
  count-up, SuperSticker scale-pop, NewMember drop-in/flash, MemberGift slide-left/
  chip-fan, GiftReceived confetti, MemberMilestone rise/growth-bar), every card
  drawing the 'made with LlamaCasty!' brand line.
- AlertOverlayLayer: true component (Commit-G pattern) — queue (cap 10, drop tail
  never stall), 33ms UI ticker, cache-first RenderFrame + UpdatePreview,
  Advance(double) as the deterministic test clock; ChatEventKind.None rows never
  enqueue.
- Wired: resolver RenderAlertBox, _alertLayer ctor + dispose, LoadLayout previews,
  Add menu item (Controls/LeftPanel.xaml), TestSessionViewModel sims tagged
  (member->NewMember, superchat->SuperChat).

Good Dog ONE integration test: AlertLayerTests (RealApp STA, real WPF raster) —
ParsePage classifies all six kinds + cadence fields; None rows enqueue nothing;
six events play pairwise-distinct moving frames then drain to null.

Gate: clean build 0 warnings; full suite 316/317, the one failure
(LayerReorderPersistenceTests.RealMouseDrag) repros on the clean tree — the
known environmental class (real-mouse-drag no-ops with a game/fullscreen window
focused).

References (OBS/overlay ecosystem):
- streamList semantics: https://developers.google.com/youtube/v3/live/docs/liveChatMessages/streamList
- OBS alert-box pattern (designated celebration zone, idle transparent): creator-chosen model

ViewModels/MainViewModel.Chat.cs

@@ -10,6 +10,7 @@ public partial class MainViewModel : ViewModelBase
 {
 
     private readonly ChatOverlayLayer _chatLayer;
+    private readonly AlertOverlayLayer _alertLayer;
 
     /// <summary>The live chat message buffer — the same collection the
     /// <see cref="ChatOverlayLayer"/> owns; here so XAML can bind it and the
@@ -22,10 +23,19 @@ public partial class MainViewModel : ViewModelBase
         => StagedScene != null
            && !Scenes.Any(s => s.Elements.OfType<Source>().Any(e => e.Type == SourceType.ChatBox));
 
+    /// <summary>The Add → Stream Alerts menu item: one alert zone per layout
+    /// (multiple zones would double-play every event).</summary>
+    public bool CanAddAlerts
+        => StagedScene != null
+           && !Scenes.Any(s => s.Elements.OfType<Source>().Any(e => e.Type == SourceType.AlertBox));
+
     public bool ShowChatInactiveMessage => !IsLive;
 
     private void OnChatMessageReceived(ChatMessage message)
-        => _chatLayer.OnMessageReceived(message, Scenes);
+    {
+        _chatLayer.OnMessageReceived(message, Scenes);
+        _alertLayer.OnMessageReceived(message, Scenes);
+    }
 
     private void UpdateChatBoxPreview(Source chatBox, IReadOnlyList<ChatMessage>? messages = null)
         => _chatLayer.UpdatePreview(chatBox, messages);
@@ -33,6 +43,12 @@ public partial class MainViewModel : ViewModelBase
     private VideoFrame? RenderChatBox(Source chatBox)
         => _chatLayer.RenderFrame(chatBox);
 
+    private void UpdateAlertBoxPreview(Source alertBox)
+        => _alertLayer.UpdatePreview(alertBox);
+
+    private VideoFrame? RenderAlertBox(Source alertBox)
+        => _alertLayer.RenderFrame(alertBox);
+
     private void StartMockChatPreview()
         => _chatLayer.StartMockPreview(Scenes);
 
@@ -41,4 +57,7 @@ public partial class MainViewModel : ViewModelBase
 
     private void UpdateActiveChatBoxPreviews()
         => _chatLayer.UpdateConfiguredBoxes(Scenes);
+
+    private void UpdateActiveAlertBoxPreviews()
+        => _alertLayer.UpdateConfiguredBoxes(Scenes);
 }