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/index.md

@@ -8,7 +8,11 @@ External-facing logic: YouTube API, persistence. See
 | `YouTubeAuthService.cs` | OAuth2 via Google: loopback callback (`http://localhost:8765/oauth2/callback`), token exchange, refresh, channel fetch. Constructor takes optional `HttpClient` + `sessionChanged` callback (test seam + save hook); session persists via `Helpers/TokenStore` (DPAPI); `ClearSession()` signs out (called by `MainViewModel.StopStream` on End Livestream) |
 | `YouTubeStreamService.cs` | Broadcast/stream management via the v3 API. **`CreateBroadcast` always sends `privacyStatus = "private"`** (ship step 7 — private-only until v1) and the one-click v3 flags (`enableAutoStart/Stop`, `enableMonitorStream=false`, `latencyPreference=low`); injectable `HttpClient? http = null` ctor seam so tests can fake the API. **`GetOrCreateReusableStreamAsync` (TASK 9, shipped 2026-08-16)** returns the channel's `ReusableStream` — lists `liveStreams?mine=true` and reuses the existing `cdn.isReusable` stream, inserting once only on first use (`resolution=variable`, `frameRate=variable`, `isReusable=true`); a stream id given to `CreateBroadcast` binds at insert via `contentDetails.boundStreamId`. RTMP URL = `ingestionAddress + "/" + streamName`. **`GetStreamHealthAsync(streamId)` (TASK 9 item 3, shipped 2026-08-16)** polls `liveStreams?part=status` → `StreamHealth` with parsed `configurationIssues[]` (NOTE 2026-09-22: the real API nests `status.healthStatus = {status, lastUpdateTimeSeconds, configurationIssues[]}` as an OBJECT — the parse reads the nested `.status` and tolerates the old flat-string shape; the object shape used to crash every poll) | **`EndBroadcastAsync` (2026-09-01; hardened 2026-09-22)** posts `liveBroadcasts/transition?broadcastStatus=complete&part=status` AFTER RTMP EOF with a pre-flight `liveBroadcasts.list` read of the broadcast's OWN `lifeCycleStatus`: only transitions from `live`/`testing`, skips already-complete broadcasts silently (2026-09-22: every blind end logged `invalidTransition` — autoStop races us to complete); `enableAutoStop` finishes every skip |
 | `StreamHealthReporter.cs` | Pure report-by-exception decision (TASK 9 item 3): `BannerFor(issues)` → `HealthIssueReport(Text?, IsError)` — null text on good/ok/noData/info-only, first warning/error issue produces its type text, error beats warning for color |
-| `YouTubeChatService.cs` | Polls `liveChat/messages`, raises `MessageReceived`; `IDisposable` |
+| `YouTubeChatService.cs` | Polls `liveChat/messages`, raises `MessageReceived`; `IDisposable`. **TASK 43 rewrite (2026-09-24):** re-arms its one-shot poll on the server's `pollingIntervalMillis` (streamList semantics — reconnect on that cadence with `nextPageToken`, clamp 1000–6000ms, `maxResults=2000`); `ParsePage` (internal static seam) decodes **all six** event types into `ChatMessage.Kind`/fields (superChat / superSticker / newSponsor / membershipGifting / giftMembershipReceived / memberMilestoneChat + milestone months best-effort regex); optional `HttpClient` ctor seam endures; `InjectSimulatedMessage` (TASK 41) preserved |
+| `ChatBoxRenderer.cs` | Chat overlay rasterizer (WPF FormattedText → BGRA `VideoFrame`), live rows + Super Chat/member styling |
+| `ChatOverlayLayer.cs` | **The chat component (Commit G):** owns the message buffer, `ChatBoxRenderer`, fade/mock timers, per-source preview renders, live `RenderFrame`; raster-on-change cache pattern (OBS text-source contract) |
+| `AlertRenderer.cs` | **The alert rasterizer (TASK 43, 2026-09-24):** renders one event's celebration animation into a BGRA `VideoFrame` — SIX DISTINCT animations (SuperChat slide-up/shine/count-up, SuperSticker scale-pop, NewMember drop-in/flash-band, MemberGift slide-left/chip-fan, GiftReceived confetti scale-in, MemberMilestone rise/growth-bar), enter/hold/exit phases, brand line on every card |
+| `AlertOverlayLayer.cs` | **The native alert-box component (TASK 43):** event queue (cap 10) + 33ms dispatch ticker + `RenderFrame` cache-first live path + `UpdatePreview`; **`Advance(double)` is the deterministic test clock** (tests never start the timer); `ChatEventKind.None` rows (sub mentions) never enqueue; idle = transparent (null frame) |
 | `LayoutStore.cs` | SQLite persistence (`Microsoft.Data.Sqlite`) at `%APPDATA%\ytLlive\ytLlive.db`; assets stored as BLOBs keyed by SHA-256 content hash; save/open layout files; schema `user_version` 6 (`Source.ClipShape`/`IsMirrored` via `ALTER TABLE` for pre-v2 DBs; v3 = singleton `Webcam` + per-scene `WebcamSceneConfig`, migrated idempotently **without backfill** — the stale `Source.DeviceId` column remains but is no longer read/written; v4 = `WebcamSceneConfig.RectWidth`/`RectHeight`, the pre-Round rect for the round-to-rect restore; v5 = `Source.IsBackdrop` + `Source.CaptureKey`, the live-capture backdrop; v6 = `Scene.HasBackdrop` — Live-only policy, one-time backfill turns Starting/BRB/Chat/Ending off + drops their backdrop sources; `MainViewModel.NormalizeBackgrounds` re-normalizes on every load — TASK 25: one locked "Background" per canonical screen (static art everywhere, DisplayCapture on Live), duplicates dropped, index 0, orphaned `BackgroundUseDefault_*`/`BackgroundPath_*` Settings keys purged at save). Settings key/value table holds app state: `MicSourceName`, `ReusableStream` (TASK 9 — the cached reusable stream's id/address/name so the pump has its RTMP URL at startup) |
 | `VideoFrame.cs` | Normalized CPU frame seam (`Width`/`Height`/tightly-packed BGRA `byte[]`) — the only pixel type the rest of the app knows about; future capture sources (screen, background-removed webcam) feed the same seam |
 | `CameraDeviceInfo.cs` | `(Id, DisplayName)` for a physical capture device |