feat(alerts): TASK 47 — alert box plays a video (built-in/custom clip) + read-time fade + message ticker

TASK 43's alert box grows a real video celebration. Per-alert IAlertClipDecoder
(ffmpeg bgra + f32le pipes, real-time paced, disposed at drain) plays the shipped
Assets/alert-default.mp4 (stamped into the Asset table at startup) unless the
creator picks their own file — path reference only, never stored in the DB; the
six AlertRenderer animations stay the fallback. ~0.3s fade rides the alpha
envelope on straight-source copies (EOF freeze-frames then fades out); audio
forwards to a new AudioMixer alert ring (8s, 48k stereo) drained at unity — no
duck, creator ruling — scaled by volume × fade. An auto-composed marquee ticker
('Funder — Super Chat · $10.00', 140px/s) scrolls top-of-frame via a
FramePump._alertTicker seam through Render/CompositeLayers, mixed into the cache
signature (dynamic overlay, never baked). New Stream Alerts section in LeftPanel.

Derivative-work references (how OBS/Streamlabs alert boxes do per-alert video):
- https://support.streamlabs.com/hc/en-us/articles/217741147-Setting-Up-Your-Streamlabs-Alerts (custom image/video per alert type + variations)
- https://obsproject.com/kb/stream-tutorial-2-alerts (alert overlay as an on-screen zone)
- https://streamlabs.com/content-hub/widgets/alert-box (per-event alert playback)

Good Dog: AlertLayerVideoTests drives a fake IAlertClipDecoder through the whole
lifecycle in one pass (custom path wins, decoder spawns/disposes, fade envelope
0→127→255, audio volume×fade, ticker scrolls, EOF fade-drain to idle). It caught
the clip branch of Advance not clearing _current before AdvanceToNext — the layer
stayed IsPlaying after drain (MyMistakes post-mortem).

Full vstest 319/319; clean build 0 warnings; scope check green.

TASKS.md

@@ -62,6 +62,7 @@
 | 44 | TEST-tab chat fix: resolve liveChatId from snippet (not contentDetails) + poll until the broadcast is live | ✅ Done (2026-09-25); full suite 318/318 | [`TASKS/task-44-test-chat-fix.md`](TASKS/task-44-test-chat-fix.md) |
 | 45 | TEST-tab chat fix #2: insert body must declare `snippet.type` (400 MISSING_REQUIRED_FIELD) | ✅ Done (2026-09-25) | [`TASKS/task-45-chat-insert-type.md`](TASKS/task-45-chat-insert-type.md) |
 | 46 | Drawers: click outside the rail closes whichever is open — TEST added to the existing Stream Settings + YPP dismiss behavior | ✅ Done (2026-09-25) | [`TASKS/task-46-drawer-click-outside-close.md`](TASKS/task-46-drawer-click-outside-close.md) |
+| 47 | Alert box video: built-in/custom alert clip (+ six-animation fallback) with read-time fade in/out + ticker + alert volume in the live mix | ✅ Done (2026-09-26) | [`TASKS/task-47-alert-videos.md`](TASKS/task-47-alert-videos.md) |
 
 ---
 
@@ -120,6 +121,17 @@
   enableAutoStart; YouTube only populates the id on live broadcasts — official GetLiveChatId.java
   sample lists broadcastStatus=active). Fixed by reading `snippet.liveChatId` and polling with a
   bounded retry after the frame pump starts; chat stays non-fatal.
+- **TASK 47 — Alert box video** — **DONE 2026-09-26** (`TASKS/task-47-alert-videos.md`):
+  the TASK 43 alert box now plays a **video** on every alert — a shipped built-in mp4
+  (`Assets/alert-default.mp4`, stamped into the `Asset` table at startup) that the creator can
+  swap for their own file via the new Stream Alerts section (path only, never stored in the DB;
+  six `AlertRenderer` animations remain the fallback). Per-alert decode via a new
+  `IAlertClipDecoder` fx (ffmpeg rawvideo bgra + f32le pipes, real-time paced, disposed at
+  drain); ~0.3s fade-in/out rides the alpha envelope + scales the audio (mixed into the live
+  stream at unity — **no duck**, a per-alert Volume slider instead); an auto-composed message
+  **ticker** ("Funder — Super Chat · $10.00") scrolls along the very top of the frame
+  (never-baked dynamic overlay threaded through SceneCompositor + FramePump). Creator rulings:
+  custom + fallback (not either/or), ticker on top, no ducking.
 
 ---