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

TASKS/task-43-native-alerts.md

@@ -0,0 +1,125 @@
+# TASK 43 — Native events & alerts (the "go" unit: chat parity + six animation alert box)
+
+> Catalog: [`TASKS.md`](../TASKS.md). Status: ✅ **SHIPPED 2026-09-24** — creator said
+> **"go"** on the combined work unit (native-chat parity + native alert box). See
+> [`ai.md`](../ai.md) → "Native events & alerts" for the architecture.
+
+## Provenance (recorded per the feature-provenance rule)
+
+- **2026-09-23, the creator asked** — "Could you use native YouTube events instead of
+  StreamElements?" (the app was designed to feed an external StreamElements alert box).
+  Research (websearch + the liveChat/messages + streamList docs, URLs in the commit
+  message) established the API facts below; the creator then ruled on product shape.
+- **Decisions the creator made in conversation (asked ONCE, then held):**
+  1. **No menus / no polls** — "I've asked you before to not use menus/polls." Decisions
+     are asked on the open floor once and then executed.
+  2. **Events = ONE animated alert box** (OBS alert-box pattern), a designated area on
+     the canvas where celebration animations play. NOT a list pane, NOT per-source.
+  3. **Six monetization events → SIX UNIQUE animations** — "not cookie-cutter": each of
+     Super Chat, Super Sticker, New Member (newSponsor), Member Gift, Gift Received,
+     Member Milestone gets its own distinct gesture, palette and layout. Each animation
+     is a free branded product-placement ad in the stream (the brand line is drawn into
+     every card) and a celebration of the supporter's investment.
+  4. **Free subscribers: NO alert** — "subscriber mention in chat is sufficient."
+     YouTube exposes no public event for free subs (API fact); a mention arrives as a
+     normal chat row and is deliberately `ChatEventKind.None` → the alert engine never
+     fires. The app keeps its opinion: none. (The third-party socket + delta-counter
+     folklore was explicitly rejected with it.)
+  5. **Viewer count: dropped entirely** — demotivating and weaponizable against the
+     creator; not even hidden.
+  6. **Native chat replaces the SE URI** — the chat box already exists in-app
+     (`SourceType.ChatBox`, fed by real `liveChat/messages` polling). The remaining
+     parity gaps closed here: the four previously-undecoded event types displayed as
+     **empty rows** in the overlay, and the poll cadence was a hardcoded 2s instead of
+     the server's `pollingIntervalMillis`.
+
+## API facts (researched + cited in commit)
+
+- Native event feed = the **same** `liveChat/messages` endpoint already polled.
+  `streamList` is a *connection semantics* description of that endpoint, not a separate
+  call: response carries `pollingIntervalMillis`; reconnect with `nextPageToken` on that
+  cadence, never faster (YouTube rejects rapid polling). The code now re-arms the
+  one-shot poll on the server's number (clamped 1000–6000ms), and `maxResults=2000`.
+  Cite: https://developers.google.com/youtube/v3/live/docs/liveChatMessages/streamList
+- Six `snippet.type` event kinds exist: `superChatEvent`, `superStickerEvent`,
+  `newSponsorEvent`, `membershipGiftingEvent`, `giftMembershipReceivedEvent`,
+  `memberMilestoneChatEvent`. Free subscriptions emit **nothing** — no API event, no
+  delta, no count (the "newSponsorEvent counts" folklore is wrong for public subs).
+- `videos.list → liveStreamingDetails.concurrentViewers` exists but was **vetoed** by
+  the creator (decision 4 above).
+
+## Product model (as shipped)
+
+- **`SourceType.AlertBox`** ("Stream Alerts") — a one-per-layout celebration zone,
+  mirroring the chat layer's gate (`CanAddAlerts` = no AlertBox exists in any scene;
+  `AddSource` refuses a second). Defaults 620×430 → 680×200. Draggable, previews like a
+  source, idle = fully transparent (nothing on stream until an event actually fires).
+- **Playback:** events from the chat feed enqueue one-at-a-time on a 33ms UI
+  dispatcher tick, play their own animation (4–5s each), then drain; back-to-back floods
+  cap the queue at 10 and drop the tail, never stalling playback. The clock is exposed
+  as `Advance(double)` so tests drive playback deterministically with no timer.
+- **The six animations** (`Services/AlertRenderer.cs`), phases enter[0–0.16] /
+  hold[0.16–0.8] / exit[0.8–1]:
+  | Kind | Gesture | Palette |
+  |---|---|---|
+  | SuperChat | slide-up + gold amount count-up + shine sweep | green `#0d3b2b→#14532d` |
+  | SuperSticker | scale-pop overshoot, tilted sticker chip, bob | purple `#4a044e→#7e22ce` |
+  | NewMember | drop-in + white flash band + gold level chip | violet `#2e1065→#5b21b6` |
+  | MemberGift | slide-left + gift-chip fan `×N` | amber `#422006→#a16207` |
+  | GiftReceived | confetti burst scale-in | green→purple `#14532d→#4c1d95` |
+  | MemberMilestone | rise-up + tenure growth bar | orange `#431407→#c2410c` |
+- Every card draws **"made with LlamaCasty!"** bottom-right — each broadcast of an event
+  is in-video product placement (creator decision: "it's a free ad each time").
+
+## Chat parity (half 1 of the unit)
+
+- `YouTubeChatService` rewritten around the real cadence + all six event parses:
+  `ParsePage(string)` (internal static seam) → `ChatPage(Messages, NextPageToken,
+  PollIntervalMs)`; fields landed on `ChatMessage`: `Kind` (`ChatEventKind`),
+  `SuperChatDisplayString`, `SuperStickerDescription`, `GiftNewMembersCount`,
+  `GifterName`, `MilestoneMonths` (best-effort `(\d+) months` regex from the milestone
+  chat text). The four formerly-empty event types now parse as real rows AND drive
+  animations. `InjectSimulatedMessage` (TASK 41) preserved.
+
+## Test (Good Dog — ONE integration test per change)
+
+`ytLive.Tests/AlertLayerTests.cs` (RealApp STA host, real WPF raster) — `AlertLayerTests`:
+1. **Parse:** a mocked 7-item feed → all six kinds classified, `None` for plain chat,
+   `pollingIntervalMillis`/`nextPageToken` honored, fields verified (incl. the
+   formerly-empty `3 memberships` / `gifted you a membership` rows).
+2. **Playback:** subscriber-mention (`None`) enqueues nothing; the six events play
+   through the queue with non-null, non-blank, *moving* frames; every event's mid-hold
+   frame is **pairwise distinct from every other** (six unique animations, pinned);
+   playback drains to transparent/null.
+
+## Files (declared scope)
+
+- `Models/YouTube.cs` (ChatEventKind + fields), `Models/Source.cs` (SourceType.AlertBox
+  + IsDraggable/Kind/DisplaySource)
+- `Services/YouTubeChatService.cs` (cadence + six-event ParsePage seam)
+- `Services/TestSessionViewModel.cs` (sims tagged: member→NewMember, superchat→SuperChat)
+- **NEW** `Services/AlertRenderer.cs`, `Services/AlertOverlayLayer.cs`
+- `ViewModels/MainViewModel.cs`, `ViewModels/MainViewModel.Chat.cs`,
+  `ViewModels/MainViewModel.Sources.cs` (wiring: `_alertLayer`, render resolver,
+  `CanAddAlerts`, menu name, shakedown `LoadLayout`, dispose)
+- `Controls/LeftPanel.xaml` (Add → Stream Alerts menu item)
+- **NEW** `ytLive.Tests/AlertLayerTests.cs`
+- Docs: `ai.md`, `TASKS.md`, this file, `Services/index.md`, `ViewModels/index.md`,
+  `Models/index.md`, `TASKS/task-03-capture-pipeline.md`, `HANDOFF.md`
+
+## Gate
+
+Clean build 0 warnings (Windows dotnet host); full suite **316/317** — the one failure
+`LayerReorderPersistenceTests.RealMouseDrag…` repros on the CLEAN tree (proved via
+`git stash`; it is a real-mouse-drag test that no-ops when a game/fullscreen window
+steals the mouse — HANDOFF-documented environmental class). Scope check green.
+
+## Open follow-ups (NOT this unit)
+
+- TASK 3 item 17 (Alerts) is DONE via native events; **item 20 stays open** — the
+  RewardEvent SQLite persistence + session-report rollup + `superChatEvents.list`
+  backfill were scoped as the capture foundation and remain queued (the shipped alert
+  path parses events directly into `ChatMessage`, which is what the report reads).
+- TASK 3 item 16 (Text source) still open.
+- Creator report still open: "I still cannot post a chat message in the TEST tab" —
+  not yet diagnosed; the TEST drawer's Mock Chat Input posts via a real insert.