chore(core): scaffold Chip8.Core project with platform and color types

src/Chip8.Core/Chip8.Core.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net6.0</TargetFramework>
+    <ImplicitUsings>disable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <RootNamespace>Chip8.Core</RootNamespace>
+    <AssemblyName>Chip8.Core</AssemblyName>
+    <LangVersion>10</LangVersion>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <InternalsVisibleTo Include="Chip8.Tests" />
+  </ItemGroup>
+
+</Project>

src/Chip8.Core/Palette.cs

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+using System;
+using System.Collections.Generic;
+
+namespace Chip8.Core;
+
+public sealed class Palette
+{
+    readonly Rgb[] colors;
+
+    public Palette(params Rgb[] colors)
+    {
+        if (colors is null || colors.Length < 2)
+        {
+            throw new ArgumentException("a palette needs at least two colours", nameof(colors));
+        }
+        this.colors = (Rgb[])colors.Clone();
+    }
+
+    public int colorCount => colors.Length;
+
+    public Rgb this[int index] => colors[index];
+
+    public Rgb resolve(int index) => colors[index < 0 ? 0 : index % colors.Length];
+
+    public IReadOnlyList<Rgb> toList() => (Rgb[])colors.Clone();
+
+    public static Palette monochrome() => new(
+        new Rgb(0x00, 0x00, 0x00),
+        new Rgb(0xFF, 0xFF, 0xFF));
+
+    public static Palette grayscale() => new(
+        new Rgb(0x00, 0x00, 0x00),
+        new Rgb(0x55, 0x55, 0x55),
+        new Rgb(0xAA, 0xAA, 0xAA),
+        new Rgb(0xFF, 0xFF, 0xFF));
+
+    public static Palette octo() => new(
+        Rgb.fromHex("#996600"),
+        Rgb.fromHex("#FFCC00"),
+        Rgb.fromHex("#FF6600"),
+        Rgb.fromHex("#662200"));
+
+    public static Palette extended16() => new(
+        Rgb.fromHex("#000000"),
+        Rgb.fromHex("#1D2B53"),
+        Rgb.fromHex("#7E2553"),
+        Rgb.fromHex("#008751"),
+        Rgb.fromHex("#AB5236"),
+        Rgb.fromHex("#5F574F"),
+        Rgb.fromHex("#C2C3C7"),
+        Rgb.fromHex("#FFF1E8"),
+        Rgb.fromHex("#FF004D"),
+        Rgb.fromHex("#FFA300"),
+        Rgb.fromHex("#FFEC27"),
+        Rgb.fromHex("#00E436"),
+        Rgb.fromHex("#29ADFF"),
+        Rgb.fromHex("#83769C"),
+        Rgb.fromHex("#FF77A8"),
+        Rgb.fromHex("#FFCCAA"));
+
+    public static Palette byName(string name) => name.ToLowerInvariant() switch
+    {
+        "mono" or "monochrome" => monochrome(),
+        "gray" or "grey" or "grayscale" => grayscale(),
+        "octo" => octo(),
+        "extended" or "extended16" or "pico8" => extended16(),
+        _ => throw new ArgumentException("unknown palette '" + name + "'", nameof(name)),
+    };
+}

src/Chip8.Core/Platform.cs

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+namespace Chip8.Core;
+
+public enum Platform
+{
+    Chip8,
+    SuperChip,
+    XoChip,
+}

src/Chip8.Core/Rgb.cs

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+using System;
+using System.Globalization;
+
+namespace Chip8.Core;
+
+public readonly struct Rgb : IEquatable<Rgb>
+{
+    public Rgb(byte r, byte g, byte b)
+    {
+        this.r = r;
+        this.g = g;
+        this.b = b;
+    }
+
+    public byte r { get; }
+    public byte g { get; }
+    public byte b { get; }
+
+    public static Rgb fromHex(string hex)
+    {
+        if (hex is null)
+        {
+            throw new ArgumentNullException(nameof(hex));
+        }
+
+        string digits = hex.StartsWith("#", StringComparison.Ordinal) ? hex.Substring(1) : hex;
+        if (digits.Length != 6)
+        {
+            throw new FormatException("expected a 6 digit hex colour like #RRGGBB, got '" + hex + "'");
+        }
+
+        byte channel(int start) => byte.Parse(digits.Substring(start, 2), NumberStyles.HexNumber, CultureInfo.InvariantCulture);
+        return new Rgb(channel(0), channel(2), channel(4));
+    }
+
+    public string toHex() =>
+        "#" + r.ToString("X2", CultureInfo.InvariantCulture)
+            + g.ToString("X2", CultureInfo.InvariantCulture)
+            + b.ToString("X2", CultureInfo.InvariantCulture);
+
+    public bool Equals(Rgb other) => r == other.r && g == other.g && b == other.b;
+
+    public override bool Equals(object? obj) => obj is Rgb other && Equals(other);
+
+    public override int GetHashCode() => (r << 16) | (g << 8) | b;
+
+    public override string ToString() => toHex();
+}