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

Models/Source.cs

@@ -13,7 +13,10 @@ public enum SourceType
     Countdown,
     WebSource,
     ChatBox,
-    MediaSource
+    MediaSource,
+    /// <summary>The designated alert zone: the six monetization events play their
+    /// unique celebration animations here (OBS alert-box equivalent, native).</summary>
+    AlertBox
 }
 
 /// <summary>Distinguishes how a media source presents its content (TASK 21).</summary>
@@ -61,11 +64,11 @@ public class Source : SceneElement
         }
     }
 
-    public override bool IsDraggable => Type is SourceType.Image or SourceType.WebSource or SourceType.ChatBox;
+    public override bool IsDraggable => Type is SourceType.Image or SourceType.WebSource or SourceType.ChatBox or SourceType.AlertBox;
 
     /// <summary>Live captures, chat, web sources and media are dynamic; static art is fixed.</summary>
     public override ElementKind Kind => Type is SourceType.DisplayCapture or SourceType.WindowCapture
-        or SourceType.ChatBox or SourceType.WebSource or SourceType.MediaSource
+        or SourceType.ChatBox or SourceType.WebSource or SourceType.MediaSource or SourceType.AlertBox
         ? ElementKind.Dynamic : ElementKind.Static;
 
     public override bool IsWebSource => Type == SourceType.WebSource;
@@ -118,7 +121,7 @@ public class Source : SceneElement
 
     public override ImageSource? DisplaySource => IsLiveCapture
         ? (VideoImageSource ?? _imageSource)
-        : (Type is SourceType.ChatBox or SourceType.WebSource or SourceType.MediaSource ? VideoImageSource : _imageSource);
+        : (Type is SourceType.ChatBox or SourceType.WebSource or SourceType.MediaSource or SourceType.AlertBox ? VideoImageSource : _imageSource);
 
     public override bool IsImageSource => Type == SourceType.Image;