refactor: move CutParameters to OpenNest.Api namespace with new properties

Relocates CutParameters from OpenNest namespace to OpenNest.Api, adds
LeadInLength and PostProcessor properties, and provides a typed Default
factory. Updates Timing.cs, the WinForms project reference, and the three
consuming forms to resolve the type from the new namespace.

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

OpenNest/Forms/EditNestForm.cs

@@ -5,6 +5,7 @@ using System.Drawing;
 using System.IO;
 using System.Linq;
 using System.Windows.Forms;
+using OpenNest.Api;
 using OpenNest.Actions;
 using OpenNest.CNC.CuttingStrategy;
 using OpenNest.Collections;