feat: add pierce clearance clamping for circle contour lead-ins

Scales down lead-ins that would place the pierce point too close to the
opposite wall of small holes. Uses quadratic solve to find the maximum
safe distance inside a clearance-reduced radius. Adds Scale() method to
all LeadIn types and applies clamping in both the strategy and the
interactive preview.

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

OpenNest.Core/CNC/CuttingStrategy/CuttingParameters.cs

@@ -21,6 +21,8 @@ namespace OpenNest.CNC.CuttingStrategy
         public LeadIn ArcCircleLeadIn { get; set; } = new NoLeadIn();
         public LeadOut ArcCircleLeadOut { get; set; } = new NoLeadOut();
 
+        public double PierceClearance { get; set; } = 0.0625;
+
         public Tab TabConfig { get; set; }
         public bool TabsEnabled { get; set; }