feat: resolve per-entity color and line type from DXF imports

Add LineTypeName to Layer and propagate resolved color/line-type
through all DXF entity conversions (Arc, Circle, Line, Spline,
Polyline, LwPolyline, Ellipse). Entities that inherit ByLayer
properties now correctly resolve to their layer's values.

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

OpenNest.Core/Geometry/Layer.cs

@@ -21,6 +21,8 @@ namespace OpenNest.Geometry
 
         public Color Color { get; set; }
 
+        public string LineTypeName { get; set; }
+
         public override string ToString()
         {
             return Name;