feat(chroma): offer to remove a green or blue screen, and key it in the decode

TASK 64 slices D and E: the feature is usable end to end.

- Add a video file → one frame is probed (new `IChromaProbe`/`FfmpegChromaProbe`, one ffmpeg
  pass at 160x90, off the UI thread) and the layer gets an offer: "Looks like a green screen —
  Remove it", with the measured colour as a swatch.
- Accepting it turns the offer into settings (Amount / Softness / Fringe) that reach ffmpeg as
  `colorkey` + `despill` + `format=bgra`, so the key is computed in the pipe and both consumers
  (the shared preview bitmap and the compositor's frame) carry the same alpha.
- Media sessions are now keyed by **path + look** (`MainViewModel.MediaSessionKey`), so two
  layers with the same settings still share one decoder, and changing the look releases the old
  session and starts the new one. Removal, load reconciliation, loop and audio settings all move
  to the same key.
- Settings persist on the `Source` row (six ALTER TABLE guards + SELECT/INSERT), so a keyed layer
  comes back keyed.
- Panel section is plain creator words: Green/Blue Screen, Amount, Softness, Fringe.

Test: `ChromaKeyApplyTests` — the pipe asks ffmpeg to key (and does not when off); a green-screen
clip is offered, accepting it starts a keyed session, the layer's resolver then reads that session
rather than the old one; the setting survives a save.

Docs: TASK 64 D/E ticked with slice F (auto-apply setting + alert clips) left open; frame-pipeline
records the decode-option design; HANDOFF rewritten. verify.sh: 544 passed, 0 failed, 0 warnings.

ViewModels/MainViewModel.Sources.cs

@@ -300,16 +300,16 @@ public partial class MainViewModel
             _webView2Manager?.Unregister(webSource.Id);
 
         // Media: the layer stops listening, and its decode session stops once no scene
-        // references the file any more (sessions are shared per path).
-        string? mediaPath = null;
+        // references the same file + look any more (TASK 64: sessions are per path + look).
+        string? mediaSessionKey = null;
         if (element is Source mediaSource && mediaSource.IsMediaSource)
         {
+            mediaSessionKey = MediaSessionKey(mediaSource);
             UnregisterMediaSource(mediaSource);
-            mediaPath = mediaSource.MediaPath;
         }
 
         _sceneGraph.RemoveElement(scene, element);
-        ReleaseMediaIfUnused(mediaPath);
+        ReleaseMediaIfUnused(mediaSessionKey);
 
         if (element is WebcamSceneConfig)
         {