1.5 PaletteEntry value type

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

src/FrameProcessor/Domain/PaletteEntry.cs

@@ -0,0 +1,118 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+using SixLabors.ImageSharp;
+
+namespace FrameProcessor.Domain;
+
+/// <summary>
+/// One palette entry mapping a display color (what the eye sees on the panel) to
+/// a device color (the RGB value firmware expects in the input PNG).
+/// Hex strings from configuration are parsed once into <see cref="Color"/> values;
+/// the entry does not retain the original strings.
+/// </summary>
+[JsonConverter(typeof(PaletteEntryJsonConverter))]
+public sealed record PaletteEntry(string Name, Color DisplayColor, Color DeviceColor);
+
+internal sealed class PaletteEntryJsonConverter : JsonConverter<PaletteEntry>
+{
+    public override PaletteEntry Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+    {
+        if (reader.TokenType != JsonTokenType.StartObject)
+        {
+            throw new JsonException("Palette entry must be a JSON object.");
+        }
+
+        string? name = null;
+        Color? displayColor = null;
+        Color? deviceColor = null;
+
+        while (reader.Read())
+        {
+            if (reader.TokenType == JsonTokenType.EndObject)
+            {
+                if (name is null)
+                {
+                    throw new JsonException("Palette entry is missing required field 'name'.");
+                }
+
+                if (displayColor is null)
+                {
+                    throw new JsonException("Palette entry is missing required field 'color'.");
+                }
+
+                if (deviceColor is null)
+                {
+                    throw new JsonException("Palette entry is missing required field 'deviceColor'.");
+                }
+
+                return new PaletteEntry(name, displayColor.Value, deviceColor.Value);
+            }
+
+            if (reader.TokenType != JsonTokenType.PropertyName)
+            {
+                throw new JsonException();
+            }
+
+            var propertyName = reader.GetString();
+            if (!reader.Read())
+            {
+                throw new JsonException("Unexpected end of palette entry.");
+            }
+
+            switch (propertyName)
+            {
+                case "name":
+                    var n = reader.GetString();
+                    if (string.IsNullOrWhiteSpace(n))
+                    {
+                        throw new JsonException("Palette entry 'name' must be a non-empty string.");
+                    }
+
+                    name = n;
+                    break;
+                case "color":
+                    displayColor = ParseHex(reader.GetString(), "color");
+                    break;
+                case "deviceColor":
+                    deviceColor = ParseHex(reader.GetString(), "deviceColor");
+                    break;
+                default:
+                    reader.Skip();
+                    break;
+            }
+        }
+
+        throw new JsonException("Unexpected end of palette entry.");
+    }
+
+    public override void Write(Utf8JsonWriter writer, PaletteEntry value, JsonSerializerOptions options)
+    {
+        writer.WriteStartObject();
+        writer.WriteString("name", value.Name);
+        writer.WriteString("color", ToRgbHex(value.DisplayColor));
+        writer.WriteString("deviceColor", ToRgbHex(value.DeviceColor));
+        writer.WriteEndObject();
+    }
+
+    private static Color ParseHex(string? s, string field)
+    {
+        if (string.IsNullOrWhiteSpace(s))
+        {
+            throw new JsonException($"Palette entry '{field}' must be a non-empty hex string.");
+        }
+
+        if (!Color.TryParseHex(s, out var color))
+        {
+            throw new JsonException($"'{s}' is not a valid hex color for '{field}'. Expected '#RRGGBB'.");
+        }
+
+        return color;
+    }
+
+    private static string ToRgbHex(Color color)
+    {
+        // ImageSharp's Color.ToHex() returns 8 characters (RRGGBBAA); SPEC writes 6-digit '#RRGGBB'.
+        var hex = color.ToHex();
+        return "#" + hex[..6];
+    }
+}