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.

Models/Source.cs

@@ -1,5 +1,6 @@
 using System.Windows.Media;
 using ytLive.Helpers;
+using ytLive.Services;
 
 namespace ytLive.Models;
 
@@ -282,6 +283,114 @@ public class Source : SceneElement
 
     public override bool IsMediaSource => Type == SourceType.MediaSource;
 
+    // ── Green/Blue Screen keying (TASK 64) ────────────────────────────────────────────
+    // Settings live on the LAYER. The decoder is keyed by (path + these settings), so two
+    // layers with the same look share one decode and two different looks get two decoders.
+
+    private bool _chromaKeyEnabled;
+    public bool ChromaKeyEnabled
+    {
+        get => _chromaKeyEnabled;
+        set
+        {
+            if (Set(ref _chromaKeyEnabled, value))
+            {
+                Raise(nameof(ChromaKey));
+                Raise(nameof(HasChromaSuggestion));
+            }
+        }
+    }
+
+    private string _chromaKeyColor = "#00B140";
+    /// <summary>The screen colour as it was shot (the detector's median), not a canonical green.</summary>
+    public string ChromaKeyColor
+    {
+        get => _chromaKeyColor;
+        set
+        {
+            if (Set(ref _chromaKeyColor, value)) Raise(nameof(ChromaKey));
+        }
+    }
+
+    private double _chromaKeySimilarity = 0.12;
+    /// <summary>Creator-facing "Amount": how far from the key colour still counts as screen.</summary>
+    public double ChromaKeySimilarity
+    {
+        get => _chromaKeySimilarity;
+        set
+        {
+            if (Set(ref _chromaKeySimilarity, Math.Clamp(value, 0, 0.5))) Raise(nameof(ChromaKey));
+        }
+    }
+
+    private double _chromaKeySoftness = 0.08;
+    /// <summary>Creator-facing "Softness": the width of the fade at the keyed edge.</summary>
+    public double ChromaKeySoftness
+    {
+        get => _chromaKeySoftness;
+        set
+        {
+            if (Set(ref _chromaKeySoftness, Math.Clamp(value, 0, 0.5))) Raise(nameof(ChromaKey));
+        }
+    }
+
+    private bool _chromaKeySpill = true;
+    /// <summary>Creator-facing "Fringe": clean the coloured rim the screen leaves on hair and edges.</summary>
+    public bool ChromaKeySpill
+    {
+        get => _chromaKeySpill;
+        set
+        {
+            if (Set(ref _chromaKeySpill, value)) Raise(nameof(ChromaKey));
+        }
+    }
+
+    private ChromaScreen _chromaScreen = ChromaScreen.Green;
+    public ChromaScreen ChromaScreen
+    {
+        get => _chromaScreen;
+        set
+        {
+            if (Set(ref _chromaScreen, value)) Raise(nameof(ChromaKey));
+        }
+    }
+
+    /// <summary>The look this layer asks its decoder for — the one place the settings become an
+    /// ffmpeg chain and a session signature.</summary>
+    public ChromaKeyOptions ChromaKey
+        => new(ChromaKeyEnabled, ChromaKeyColor, ChromaKeySimilarity, ChromaKeySoftness, ChromaKeySpill, ChromaScreen);
+
+    private ChromaKeySuggestion _chromaSuggestion;
+    /// <summary>The detector's offer for this clip. Transient by design — never persisted, because
+    /// it is a hint about a file, not a setting the creator made.</summary>
+    public ChromaKeySuggestion ChromaSuggestion
+    {
+        get => _chromaSuggestion;
+        set
+        {
+            _chromaSuggestion = value;
+            Raise(nameof(ChromaSuggestion));
+            Raise(nameof(HasChromaSuggestion));
+            Raise(nameof(ChromaSuggestionText));
+            Raise(nameof(HasChromaSuggestionColor));
+        }
+    }
+
+    /// <summary>True while there is an offer the creator has not accepted.</summary>
+    public bool HasChromaSuggestion => !ChromaKeyEnabled && ChromaSuggestion.Screen != ChromaScreen.None;
+
+    public string ChromaSuggestionText => ChromaSuggestion.Screen switch
+    {
+        ChromaScreen.Green => "Looks like a green screen.",
+        ChromaScreen.Blue => "Looks like a blue screen.",
+        _ => string.Empty,
+    };
+
+    /// <summary>The detected colour, for the swatch next to the offer.</summary>
+    public string ChromaSuggestionColor => ChromaSuggestion.ColorHex ?? "#00B140";
+
+    public bool HasChromaSuggestionColor => ChromaSuggestion.ColorHex != null;
+
     // Alert video config (TASK 47): the shipped default clip lives in the Asset
     // table (seeded like image assets); the creator's replacement is a FILE PATH
     // reference only — never into the DB (creator ruling, 2026-09-26). Runtime