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).

ai.md

@@ -1630,16 +1630,34 @@ published decisions (recorded in `TASKS/task-43-native-alerts.md`):
   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` +
+- **Alert ticker (announcement strip):** `AlertTickerFrame` (in the layer) composes
+  "Author — Kind · amount" and hands it to `Services/Compositor/AlertTickerRenderer.cs` —
+  a 1920×48 strip rasterized **on the UI thread once per text** (cached, unpremultiplied
+  straight-alpha pill) then positioned by pure byte-math per call, 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`.
+  - **Three display methods** (`Source.AlertDisplayMethod`, DB column, panel "Display"
+    selector): `TickerScroll` (marquee), `Flash` (0.5s on / 0.5s off), `Solid` (centred,
+    still). The marquee is paced in **reads per alert** — `TickerReadsPerAlert = 3`
+    passes inside the alert's own length, speed derived from that — **never px/s**; the
+    old fixed `140px/s` took ~17s per pass, so a 10s alert showed the message once.
+    Same convention as the incumbent (Streamlabs: "Alert Duration" + "Text Delay", not a
+    scroll-speed slider). Flash's off half returns null.
+  - **A global overlay needs its own preview element, not a `Source`.** The strip is
+    master-width, so it can't ride a per-element `Image`; a frame-pump-only producer
+    reaches the stream and never the creator's preview. `AlertOverlayLayer` takes a
+    `tickerPreviewSink` and publishes from `RefreshAlertPreviews()` (UI thread only —
+    the pump thread must never touch a `WriteableBitmap`); the VM writes it into one
+    reused `WriteableBitmap` (`AlertTickerImageSource`/`AlertTickerVisible`) bound to a
+    global `AlertTickerElement` in `PreviewPane.xaml`, mirroring `SocialBarElement`.
+  - **Two measurement traps here:** a seamless marquee never goes blank (a wrapped copy
+    enters as the pill clears), so count a pass by the pill's leading edge resetting, not
+    by an empty frame; and the run is phase-started half a frame in, or the first
+    published frame is blank (the pill sits exactly off the right edge).
 - **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