1.6 DitherAlgorithm value type

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/FrameProcessor/Domain/DitherAlgorithm.cs

@@ -0,0 +1,40 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace FrameProcessor.Domain;
+
+[JsonConverter(typeof(DitherAlgorithmJsonConverter))]
+public enum DitherAlgorithm
+{
+    FloydSteinberg,
+    Atkinson,
+    Stucki,
+    Jarvis,
+}
+
+internal sealed class DitherAlgorithmJsonConverter : JsonConverter<DitherAlgorithm>
+{
+    public override DitherAlgorithm Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+    {
+        var s = reader.GetString();
+        return s switch
+        {
+            "floyd-steinberg" => DitherAlgorithm.FloydSteinberg,
+            "atkinson" => DitherAlgorithm.Atkinson,
+            "stucki" => DitherAlgorithm.Stucki,
+            "jarvis" => DitherAlgorithm.Jarvis,
+            _ => throw new JsonException(
+                $"'{s}' is not a valid dither algorithm. Expected 'floyd-steinberg', 'atkinson', 'stucki', or 'jarvis'."),
+        };
+    }
+
+    public override void Write(Utf8JsonWriter writer, DitherAlgorithm value, JsonSerializerOptions options)
+        => writer.WriteStringValue(value switch
+        {
+            DitherAlgorithm.FloydSteinberg => "floyd-steinberg",
+            DitherAlgorithm.Atkinson => "atkinson",
+            DitherAlgorithm.Stucki => "stucki",
+            DitherAlgorithm.Jarvis => "jarvis",
+            _ => throw new JsonException($"Unknown dither algorithm: {value}."),
+        });
+}