feat(world-gen): implement noise-threshold cave pass

VoxelEngine.Tests/WorldGen/CaveCarverTests.cs

@@ -35,4 +35,84 @@ public class CaveCarverTests
         for (int z = 0; z < Chunk.Size; z++)
             Assert.Equal(Blocks.Air.Id, c[x, y, z]);
     }
+
+    [Fact]
+    public void NoiseCaves_OnlyConvertsSolidToAir_NeverWaterNeverOtherBlocks()
+    {
+        var carver = new CaveCarver(seed: 42, Blocks, Cfg);
+        Chunk c = MakeStoneChunk();
+
+        // Plant some water and air voxels we expect untouched.
+        c[1, 1, 1] = Blocks.Water.Id;
+        c[2, 2, 2] = Blocks.Air.Id;
+
+        carver.CarveNoiseCaves(c, new Vector3i(0, 4, 0));
+
+        Assert.Equal(Blocks.Water.Id, c[1, 1, 1]); // water survived
+        Assert.Equal(Blocks.Air.Id,   c[2, 2, 2]); // air survived
+
+        // Spot-check that everywhere else is either Stone or Air (no other blocks).
+        for (int x = 0; x < Chunk.Size; x++)
+        for (int y = 0; y < Chunk.Size; y++)
+        for (int z = 0; z < Chunk.Size; z++)
+        {
+            ushort id = c[x, y, z];
+            Assert.True(
+                id == Blocks.Stone.Id || id == Blocks.Air.Id || id == Blocks.Water.Id,
+                $"unexpected block id {id} at ({x},{y},{z})");
+        }
+    }
+
+    [Fact]
+    public void NoiseCaves_IsDeterministic_ForSameSeedAndChunkPos()
+    {
+        var carver1 = new CaveCarver(seed: 1234, Blocks, Cfg);
+        var carver2 = new CaveCarver(seed: 1234, Blocks, Cfg);
+        Chunk a = MakeStoneChunk();
+        Chunk b = MakeStoneChunk();
+
+        carver1.CarveNoiseCaves(a, new Vector3i(2, 4, 3));
+        carver2.CarveNoiseCaves(b, new Vector3i(2, 4, 3));
+
+        for (int x = 0; x < Chunk.Size; x++)
+        for (int y = 0; y < Chunk.Size; y++)
+        for (int z = 0; z < Chunk.Size; z++)
+            Assert.Equal(a[x, y, z], b[x, y, z]);
+    }
+
+    [Fact]
+    public void NoiseCaves_RespectsBedrockFloor_DoesNotCarveBelowY1()
+    {
+        // ChunkPos.Y = 0 means worldY ranges 0..15. We require Y=0 stays solid.
+        var carver = new CaveCarver(seed: 42, Blocks, Cfg);
+        Chunk c = MakeStoneChunk();
+
+        carver.CarveNoiseCaves(c, new Vector3i(0, 0, 0));
+
+        for (int x = 0; x < Chunk.Size; x++)
+        for (int z = 0; z < Chunk.Size; z++)
+            Assert.Equal(Blocks.Stone.Id, c[x, 0, z]); // worldY = 0 is bedrock
+    }
+
+    [Fact]
+    public void NoiseCaves_SeamContinuity_AdjacentChunksAgreeAtBoundary()
+    {
+        var carver = new CaveCarver(seed: 7, Blocks, Cfg);
+        Chunk c1 = MakeStoneChunk();
+        Chunk c2 = MakeStoneChunk();
+
+        carver.CarveNoiseCaves(c1, new Vector3i(0, 4, 0));
+        carver.CarveNoiseCaves(c2, new Vector3i(1, 4, 0));
+
+        int airAtSeamC1 = 0, airAtSeamC2 = 0;
+        for (int y = 1; y < Chunk.Size; y++)
+        for (int z = 0; z < Chunk.Size; z++)
+        {
+            if (c1[15, y, z] == Blocks.Air.Id) airAtSeamC1++;
+            if (c2[0,  y, z] == Blocks.Air.Id) airAtSeamC2++;
+        }
+        // Sanity: noise threshold = -0.3 yields some carved cells in each.
+        Assert.True(airAtSeamC1 > 0, "c1 seam column had no air — carve too sparse?");
+        Assert.True(airAtSeamC2 > 0, "c2 seam column had no air — carve too sparse?");
+    }
 }