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/YouTube.cs

@@ -18,6 +18,21 @@ public record ReusableStream(string Id, string IngestionAddress, string StreamNa
     public string RtmpUrl => $"{IngestionAddress}/{StreamName}";
 }
 
+/// <summary>The six monetization events YouTube's live chat feed emits (the ONLY
+/// native event stream there is — free subscribers produce no event). Each kind
+/// drives its own celebration animation in the AlertBox. <c>None</c> = a normal
+/// chat row: it never enqueues an alert (subscriber mentions stay chat rows).</summary>
+public enum ChatEventKind
+{
+    None,
+    SuperChat,
+    SuperSticker,
+    NewMember,
+    MemberGift,
+    GiftReceived,
+    MemberMilestone,
+}
+
 public class ChatMessage
 {
     public string Id { get; init; } = Guid.NewGuid().ToString();
@@ -29,9 +44,28 @@ public class ChatMessage
     public bool IsSuperChat { get; set; }
     public double SuperChatAmount { get; set; }
     public string? SuperChatCurrency { get; set; }
+
+    /// <summary>YouTube's own pre-rendered amount ("$12.34") — preferred over
+    /// recomposing amount+currency ourselves, which mangles non-dollar locales.</summary>
+    public string? SuperChatDisplayString { get; set; }
+
     public bool IsMember { get; set; }
     public string? MembershipLevel { get; set; }
 
+    public ChatEventKind Kind { get; set; }
+
+    public string? SuperStickerDescription { get; set; }
+
+    /// <summary>membershipGiftingEvent: how many memberships were bought.</summary>
+    public int GiftNewMembersCount { get; set; }
+
+    /// <summary>giftMembershipReceivedEvent: who gifted this one.</summary>
+    public string? GifterName { get; set; }
+
+    /// <summary>memberMilestoneChatEvent: streak months (0 = YouTube didn't
+    /// expose a count — the animation falls back to the message text).</summary>
+    public int MilestoneMonths { get; set; }
+
     /// <summary>TASK 41: the Test Stream dock injects synthetic member / SuperChat /
     /// subscriber events locally (YouTube's insert API only supports text), so the
     /// buffer can label them honestly — simulated events never reach YouTube.</summary>