TASK 47 take four: alert ticker visible in the preview + 3 display methods

The creator confirmed the clip fix ("the video plays now"), then asked where the
scrolling text was — it was invisible, for a structural reason. AlertTickerFrame
existed only as a frame-pump callback blitted into the OUTPUT; PreviewPane.xaml had
no element for it, because the strip is master-width and global, not a Source, so it
cannot ride a per-element Image. Nothing was wrong in the renderer: there was no
consumer in the preview. Same class of defect as the missing IsAlertBox trigger, one
layer up (MyMistakes RULE 5/6).

- tickerPreviewSink on AlertOverlayLayer, published from RefreshAlertPreviews() so it
  is always the UI thread; MainViewModel.AlertTicker writes it into one reused
  WriteableBitmap bound to a new global AlertTickerElement, mirroring SocialBarElement.
- Source.AlertDisplayMethod + panel "Display" selector: TickerScroll / Flash / Solid.
  Flash pulses 0.5s on / 0.5s off for the whole alert; Solid is centred and still.
- The marquee was also unreadable: a fixed 140px/s took ~17s per pass, so a 10s alert
  showed the text once, entering from the right and never crossing. Paced in reads per
  alert instead (TickerReadsPerAlert = 3 inside the alert's own length, speed derived
  from it) — never px/s. Research (websearch: how do OBS/Streamlabs/StreamElements
  alert boxes present announcement timing?) settled the unit: Streamlabs exposes "Alert
  Duration: choose how long your alert stays on your stream" and "Text Delay", never a
  scroll-speed slider (https://support.streamlabs.com/hc/en-us/articles/52499995174299-Setting-up-Your-Streamlabs-Alerts).
  Run is phase-started half a frame in so the first frame isn't blank.
- Persistence: AlertDisplayMethod INTEGER NOT NULL DEFAULT 0 via the idempotent
  table_info migration, appended LAST in the SELECT because the Source reader is
  positional (GetInt32(32..34)) — a mid-list insert would silently shift a neighbour.

Tests: 15 new facts (suite 339/339). RealApp STA host: the pane draws the strip and
collapses at alert end; the layer publishes a real 1920x48 frame for all three methods
and nothing when the ticker is off; three passes counted in 10s by the pill's leading
edge resetting (a seamless marquee never blanks, so an empty frame cannot count a pass);
Solid byte-identical at every moment; Flash on for half of each second; the panel shows
and writes back the choice; the DB round-trips all three alert fields together.

Incidental finding: a bound ItemsSource ComboBox in LeftPanel.xaml broke
LayerReorderPersistenceTests.RealMouseDrag (that test injects PHYSICAL mouse input, so a
load-time re-measure moves the rows out from under the cursor). Rewritten as inline
ComboBoxItems, the shape the chat Font selector already uses in that panel. Recorded as
MyMistakes RULE (8).

TASKS.md

@@ -62,7 +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) |
+| 47 | Alert box video: built-in/custom alert clip (+ six-animation fallback) with read-time fade in/out + ticker (now in the preview too, 3 display methods) + alert volume in the live mix | ✅ Done (2026-09-26) | [`TASKS/task-47-alert-videos.md`](TASKS/task-47-alert-videos.md) |
 
 ---
 
@@ -121,6 +121,13 @@
   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 take four (2026-09-26)** — the announcement strip was **output-only** (a
+  frame-pump callback with no preview consumer, since a master-width global overlay cannot
+  ride a per-element `Image`) and the marquee was paced at a fixed `140px/s` (~17s per
+  pass, so a 10s alert showed the text once). Now: a `tickerPreviewSink` publishes it to a
+  global `AlertTickerElement` in `PreviewPane.xaml` (mirroring `SocialBarElement`), and the
+  creator picks **Scroll / Flash / Solid** in the panel, with Scroll paced in *reads per
+  alert* (3 inside a 10s alert) rather than px/s. 15 new facts; suite 339/339.
 - **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