feat(core): add Vertex.TerrainLayer + LandblockMesh layer map

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/AcDream.Core/Terrain/Vertex.cs

@@ -2,4 +2,8 @@ using System.Numerics;
 
 namespace AcDream.Core.Terrain;
 
-public readonly record struct Vertex(Vector3 Position, Vector3 Normal, Vector2 TexCoord);
+public readonly record struct Vertex(
+    Vector3 Position,
+    Vector3 Normal,
+    Vector2 TexCoord,
+    uint TerrainLayer);