feat(world-gen): move cave ceiling and dark bands to new sea level (256)

Core/Configuration.cs

@@ -61,12 +61,12 @@ public sealed class Configuration
     public float WormMinRadius                { get; set; } = 1.5f;
     public float WormMaxRadius                { get; set; } = 3.5f;
     // Ceiling: caves carve only at or below this world-Y ("no caves above").
-    // Default 127 = world top (WorldHeight - 1) → no effective ceiling. Lower it
-    // to keep caves away from the surface (removes surface cave mouths).
-    public int   MinCaveY                     { get; set; } = 127;
+    // Default 256 = SeaLevel → caves stay below sea level, keeping the taller
+    // surface terrain solid. Raise toward WorldHeight-1 to allow surface caves.
+    public int   MinCaveY                     { get; set; } = 256;
 
     // -- Cave lighting (fog-distance darkness) --
-    public float CaveDarkStartY    { get; set; } = 72f;   // WorldConstants.SeaLevel + 8
-    public float CaveDarkEndY      { get; set; } = 32f;   // WorldConstants.SeaLevel - 32
+    public float CaveDarkStartY    { get; set; } = 264f;  // WorldConstants.SeaLevel + 8
+    public float CaveDarkEndY      { get; set; } = 224f;  // WorldConstants.SeaLevel - 32
     public float CaveMinBrightness { get; set; } = 0.15f;
 }

VoxelEngine.Tests/ConfigurationTests.cs

@@ -70,8 +70,8 @@ public class ConfigurationTests
         Assert.Equal(3.5f,  c.WormMaxRadius);
 
         // Lighting (fog)
-        Assert.Equal(72f,   c.CaveDarkStartY);    // SeaLevel (64) + 8
-        Assert.Equal(32f,   c.CaveDarkEndY);      // SeaLevel (64) - 32
+        Assert.Equal(264f,  c.CaveDarkStartY);    // SeaLevel (256) + 8
+        Assert.Equal(224f,  c.CaveDarkEndY);      // SeaLevel (256) - 32
         Assert.Equal(0.15f, c.CaveMinBrightness);
     }
 
@@ -83,6 +83,6 @@ public class ConfigurationTests
         Assert.Equal(1, c.SurfaceShellSkirtBelow);
         Assert.Equal(0.7f, c.FogStartFraction);
         Assert.Equal(new OpenTK.Mathematics.Vector3(0.45f, 0.65f, 0.95f), c.SkyColor);
-        Assert.Equal(127, c.MinCaveY);
+        Assert.Equal(256, c.MinCaveY);
     }
 }