feat: add option to round lead-in angles for circle holes

Snaps lead-in angles on ArcCircle contours to a configurable
increment (default 5°), reducing unique hole variations from
infinite to 72 max. Rounding happens upstream in EmitContour
so the PlateView and post output stay in sync.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest.Core/CNC/CuttingStrategy/CuttingParameters.cs

@@ -23,6 +23,9 @@ namespace OpenNest.CNC.CuttingStrategy
 
         public double PierceClearance { get; set; } = 0.0625;
 
+        public bool RoundLeadInAngles { get; set; }
+        public double LeadInAngleIncrement { get; set; } = 5.0;
+
         public double AutoTabMinSize { get; set; }
         public double AutoTabMaxSize { get; set; }