feat: switch from netDxf to ACadSharp for DXF operations

Replace vendored netDxf git submodule with ACadSharp NuGet package
(v3.1.32). This aligns the DXF dependency with the parent project.

API migration: DxfDocument→CadDocument, Vector3→XYZ, Vector2→XY,
MText.Position→MText.InsertPoint, Line.Linetype→Line.LineType,
DxfDocument.Load()→DxfReader, doc.Save()→DxfWriter.

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

.gitmodules

@@ -1,3 +0,0 @@
-[submodule "netDxf"]
-	path = netDxf
-	url = https://github.com/haplokuon/netDxf.git