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.

Services/MediaVideoSourceManager.cs

@@ -41,7 +41,8 @@ public sealed class MediaVideoSourceManager : IDisposable
         }
     }
 
-    private readonly Func<string, IMediaFrameSource?> _sourceFactory;
+    // (file path, the layer's look) → decoder. The SESSION key is separate: path + look.
+    private readonly Func<string, ChromaKeyOptions?, IMediaFrameSource?> _sourceFactory;
     private readonly Dispatcher? _uiDispatcher;
     private readonly Func<string, bool>? _loopProvider;
     private readonly Dictionary<string, MediaSession> _sessions = new();
@@ -58,7 +59,7 @@ public sealed class MediaVideoSourceManager : IDisposable
     public event Action<string, AudioSample>? AudioAvailable;
 
     public MediaVideoSourceManager(
-        Func<string, IMediaFrameSource?> sourceFactory,
+        Func<string, ChromaKeyOptions?, IMediaFrameSource?> sourceFactory,
         Dispatcher? uiDispatcher = null,
         Func<string, bool>? loopProvider = null)
     {
@@ -67,8 +68,14 @@ public sealed class MediaVideoSourceManager : IDisposable
         _loopProvider = loopProvider;
     }
 
-    /// <summary>Increments the refcount for a file, starting the decoder the first time.</summary>
-    public async Task<bool> AcquireAsync(string key)
+    /// <summary>Increments the refcount for a file, starting the decoder the first time.
+    /// <paramref name="keyOptions"/> is the layer's look (TASK 64): the caller keys sessions by
+    /// path + look, so two looks on one file are two decoders and identical looks share one.</summary>
+    public async Task<bool> AcquireAsync(string key, ChromaKeyOptions? keyOptions = null)
+        => await AcquireAsync(key, key, keyOptions).ConfigureAwait(false);
+
+    /// <summary>Acquires by session key, decoding <paramref name="path"/> with the given look.</summary>
+    public async Task<bool> AcquireAsync(string key, string path, ChromaKeyOptions? keyOptions = null)
     {
         if (string.IsNullOrWhiteSpace(key)) return false;
 
@@ -87,7 +94,7 @@ public sealed class MediaVideoSourceManager : IDisposable
                 IMediaFrameSource? source;
                 try
                 {
-                    source = _sourceFactory(key);
+                    source = _sourceFactory(path, keyOptions);
                 }
                 catch (Exception ex)
                 {