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

Services/TestSessionViewModel.cs

@@ -184,7 +184,9 @@ public sealed class TestSessionViewModel : ViewModelBase
 
     /// <summary>Subscriptions aren't chat events on YouTube, so a "subscriber" row
     /// is a plain text-shaped message carrying the count — honest: the chat overlay
-    /// shows it as a regular row, which is exactly what a sub looks like in chat.</summary>
+    /// shows it as a regular row, which is exactly what a sub looks like in chat.
+    /// It carries <see cref="ChatEventKind.None"/> on purpose: no alert animation
+    /// fires for free subs — the chat mention is the announcement (creator ruling).</summary>
     private ChatMessage SimulatedSubscriber()
     {
         var author = NextViewer();
@@ -197,6 +199,7 @@ public sealed class TestSessionViewModel : ViewModelBase
         var message = NewSimulated(author, $"🎉 {author} became a member!");
         message.IsMember = true;
         message.MembershipLevel = "New member";
+        message.Kind = ChatEventKind.NewMember;
         return message;
     }
 
@@ -207,6 +210,8 @@ public sealed class TestSessionViewModel : ViewModelBase
         message.IsSuperChat = true;
         message.SuperChatAmount = 5.00;
         message.SuperChatCurrency = "$";
+        message.SuperChatDisplayString = "$5.00";
+        message.Kind = ChatEventKind.SuperChat;
         return message;
     }