Add ToString overrides for HalfEdgeMesh handle types

I kept ToString() to just the index so it stays a low-level identity string. Higher-level tools can add whatever labeling or context they need

Author: Aleph0x

engine/Sandbox.Engine/Scene/Components/Mesh/HalfEdgeMesh/HalfEdgeMesh.cs

@@ -72,6 +72,9 @@ public HalfEdgeHandle Edge
 		get => new( Mesh is null ? -1 : Mesh[this].Edge, Mesh );
 		set => Mesh?.SetVertexEdge( this, value );
 	}
+
+	public override string ToString()
+		=> Index >= 0 ? $"{Index}" : "Invalid Vertex";
 }
 
 public sealed record FaceHandle : IHandle
@@ -93,6 +96,9 @@ public HalfEdgeHandle Edge
 		get => new( Mesh is null ? -1 : Mesh[this].Edge, Mesh );
 		set => Mesh?.SetFaceEdge( this, value );
 	}
+
+	public override string ToString()
+		=> Index >= 0 ? $"{Index}" : "Invalid Face";
 }
 
 public sealed record HalfEdgeHandle : IHandle
@@ -132,6 +138,9 @@ public FaceHandle Face
 		get => new( Mesh is null ? -1 : Mesh[this].Face, Mesh );
 		set => Mesh?.SetEdgeFace( this, value );
 	}
+
+	public override string ToString()
+		=> Index >= 0 ? $"{Index}" : "Invalid Edge";
 }
 
 internal sealed partial class Mesh