feat: add BiomeMap with per-cell climate classification

VoxelEngine.Tests/WorldGen/BiomeMapTests.cs

@@ -0,0 +1,54 @@
+using VoxelEngine.World.Blocks;
+using VoxelEngine.World.WorldGen;
+using Xunit;
+
+namespace VoxelEngine.Tests.WorldGen;
+
+public class BiomeMapTests
+{
+    private static (BiomeMap map, BiomeRegistry biomes) Build(int seed = 42)
+    {
+        var blocks = new BlockRegistry();
+        var biomes = new BiomeRegistry(seed, blocks);
+        var noise = new NoiseSource(seed);
+        var cells = new VoronoiCells(seed, 96f);
+        return (new BiomeMap(noise, cells, biomes), biomes);
+    }
+
+    [Fact]
+    public void Classify_HighElevation_IsMountains()
+    {
+        var (_, biomes) = Build();
+        Assert.Same(biomes.Mountains, BiomeMap.Classify(0f, 0.5f, 0f, biomes));
+    }
+
+    [Fact]
+    public void Classify_HotLowElevation_IsDesert()
+    {
+        var (_, biomes) = Build();
+        Assert.Same(biomes.Desert, BiomeMap.Classify(0.5f, 0f, 0f, biomes));
+    }
+
+    [Fact]
+    public void Classify_TemperateWooded_IsForest()
+    {
+        var (_, biomes) = Build();
+        Assert.Same(biomes.Forest, BiomeMap.Classify(0f, 0f, 0.5f, biomes));
+    }
+
+    [Fact]
+    public void Classify_ColdLow_IsPlains()
+    {
+        var (_, biomes) = Build();
+        Assert.Same(biomes.Plains, BiomeMap.Classify(-0.9f, 0f, 0f, biomes));
+    }
+
+    [Fact]
+    public void BiomeAt_IsConstantWithinACell()
+    {
+        var (map, _) = Build();
+        IBiome a = map.BiomeAt(500f, 500f);
+        IBiome b = map.BiomeAt(500.01f, 500.01f); // same cell
+        Assert.Same(a, b);
+    }
+}