TASK 47: draw the alert clip — the preview Image was Collapsed for AlertBox

Four builds (89/90 + two) burned proving the alert video was PERFECT: real h264
1280x720, 240 frames decoded, alert audio in the live mix, and the shipped asset
byte-identical (md5 0ee1f496…) to the creator's llamacasty-dancingLlama-thankyou.mp4
with every sampled frame full bright content. Build 90's new diagnostics then showed
the frame reaching BOTH consumers every second for the whole clip —
`Alert preview: frame=680x200 a255` and `Output resolver: frame 680x200
playing=True` — an opaque, correctly-sized frame, handed over and never seen.

Root cause: Controls/PreviewPane.xaml keeps the per-element Image Collapsed unless a
DataTrigger fires, and there was no IsAlertBox trigger (only IsImageSource / IsChatBox /
IsWebSource / IsWebcam). The alert box's Image was therefore Collapsed forever. Chat
boxes rendered because they HAVE a trigger — that asymmetry is the whole clue, and it
is why removing/re-adding the layer could never fix it.

Fix: an IsAlertBox DataTrigger beside the IsChatBox one.

Two Good Dog tests, because the existing fakes had been hiding this:
- AlertBoxPreviewVisibilityTests (red `Expected: Visible / Actual: Collapsed`, now
  green) drives the real MainWindow + PreviewPane and asserts the bound Image is
  Visible — the first alert test that crosses the XAML at all.
- AlertClipOutputTests is the first test in the repo to run a REAL codec: pinned ffmpeg
  generates a clip, the real AlertClipDecoder + AlertOverlayLayer + SceneCompositor
  composite it, and the box rect must be a colourful picture that DIFFERS from idle. It
  passed while the feature was broken in the app — which is exactly why it was needed:
  it exonerated decode+layer+compositor and pointed the hunt at the last hop.

Also keeps the two once-per-second alert diagnostics (resolver + preview) that settled
it, pending the creator's call on whether to keep them.

Notes: the pinned BtbN ffmpeg has no libx264, so the test clip is -c:v mpeg4.
SceneCompositor.Render fills its base with opaque black, so the test compares against an
idle render rather than counting non-zero bytes. MediaSource has the same missing trigger
in PreviewPane.xaml — left unfixed as out of scope, noted in HANDOFF.

Full suite 324/324. Lesson recorded in MyMistakes.md rule (5): a producer that hands over
a correct frame has still delivered nothing — when output arrives and the user still sees
nothing, stop auditing the producer and audit the consumer's VISIBILITY.

Controls/PreviewPane.xaml

@@ -113,6 +113,13 @@
                                                                     <DataTrigger Binding="{Binding IsChatBox}" Value="True">
                                                                          <Setter Property="Visibility" Value="Visible"/>
                                                                      </DataTrigger>
+                                                                     <!-- TASK 47: the alert box's frame (the decoded clip) is written onto
+                                                                          VideoImageSource exactly like the chat box's, so it needs the same
+                                                                          trigger. Without it the Image stayed Collapsed and every alert was
+                                                                          invisible in the preview while the decode logged healthy (2026-09-26). -->
+                                                                     <DataTrigger Binding="{Binding IsAlertBox}" Value="True">
+                                                                         <Setter Property="Visibility" Value="Visible"/>
+                                                                     </DataTrigger>
                                                                      <DataTrigger Binding="{Binding IsWebSource}" Value="True">
                                                                          <Setter Property="Visibility" Value="Visible"/>
                                                                          <Setter Property="Stretch" Value="Fill"/>