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.

ai.md

@@ -1616,3 +1616,31 @@ published decisions (recorded in `TASKS/task-43-native-alerts.md`):
   **never enqueue** — per the creator ruling, a subscriber mention is sufficient; there is
   no free-sub alert (YouTube emits none) and no viewer count (vetoed — demotivating,
   weaponizable).
+- **Alert box VIDEO (TASK 47, 2026-09-26):** since TASK 47 the box plays a clip on every
+  alert instead of (only) an animation — `Enqueue`→`BeginClip` spawns a **per-play**
+  `IAlertClipDecoder` (ffmpeg `rawvideo bgra -vf scale=W:H` frame pipe + `f32le` audio
+  pipe, both real-time paced via `RawVideoFrameReader`; **not** the refcounted
+  `MediaVideoSourceManager` — alerts are one-shot plays that must be disposed at drain).
+  Custom clip wins per-source over the built-in ONLY when `AlertUseDefaultVideo==false`
+  AND the file exists; the shipped `Assets/alert-default.mp4` is stamped into the `Asset`
+  table at startup (`AlertDefaultVideoKey` setting; the prune keeps it via UNION) and
+  read-at-play; any decode failure falls back to the six `AlertRenderer` animations
+  (deliberate, not an error). Fades in/out via **`FadeDurationSeconds = 0.30`**
+  (`AlertOverlayLayer` const): fade-in by `elapsed` on straight-alpha frame copies;
+  EOF → freeze-frame → fade-out → drain (the clip branch of `Advance` **clears
+  `_current` before `AdvanceToNext`** — the miss that the TASK 47 test caught). Audio
+  forwards to the mixer sink scaled by `volume × fade`.
+- **Alert ticker:** `AlertTickerFrame` (in the layer) composes "Author — Kind · amount"
+  and hands it to `Services/Compositor/AlertTickerRenderer.cs` — a 1920×48 marquee strip
+  rasterized **on the UI thread once per text** (cached, unpremultiplied straight-alpha
+  pill) then scrolled by pure byte-math per call (`140px/s`, repeat-gap wrap), so the
+  pump calls it from its own thread with no locking. **It is a dynamic overlay, NEVER
+  baked** (the social bar IS baked into `BakeStaticBase`): `SceneCompositor.Render` +
+  `CompositeLayers` take a trailing `tickerFrame` (blitted last at 0,0) threaded from
+  `FramePump._alertTicker` (`Func<VideoFrame?>` ctor seam) through
+  `RenderScene`/`RenderFull` and **mixed into `BuildFullRenderSignature`** so a scroll
+  changes the cache key. Toggled by `AlertShowTicker`.
+- **Alert audio under the master limiter:** `AudioMixer.EnqueueAlertAudio` writes a
+  dedicated 8s stereo-48k ring (`AlertBufferSeconds`), drained in `FillAndMix` before the
+  limiter and added at **unity — never ducked** (creator ruling: "don't lower my game
+  audio for a tip"); `StartLive` clears it. Per-alert loudness = the Volume slider.