feat: improve DXF export and cross-platform file handling

- PepSettings: add DrawingsDirectory property for DXF endpoint config
- DrawingDxfExporter: support subprogram calls, skip non-cut linear moves,
  remove spurious radians-to-degrees conversion on arc angles, set AC1018 DXF version
- DrawingReader: handle .loop-info/.cadsnapshot entries gracefully, derive
  DrawingInfo from loop names when .info entry is absent, use FileShare.Read
- DrawingInfoReader: use TryParse instead of Parse for material number
- Drawing: make GetLoopName public (used by DrawingDxfExporter)
- ZipHelper: use FileShare.Read to allow concurrent readers on Windows/Linux
- NestFilterData: skip status filter when value is "All"

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

PepLib.Core/Models/Drawing.cs

@@ -76,7 +76,7 @@ namespace PepLib.Models
             return null;
         }
 
-        private string GetLoopName(int loopId)
+        public string GetLoopName(int loopId)
         {
             return string.Format("{0}.loop-{1}", Info.Name, loopId.ToString().PadLeft(3, '0'));
         }