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/LeadIns/LineLineLeadIn.cs

@@ -40,5 +40,8 @@ namespace OpenNest.CNC.CuttingStrategy
                 midX + Length1 * System.Math.Cos(firstAngle),
                 midY + Length1 * System.Math.Sin(firstAngle));
         }
+
+        public override LeadIn Scale(double factor) =>
+            new LineLineLeadIn { Length1 = Length1 * factor, ApproachAngle1 = ApproachAngle1, Length2 = Length2 * factor, ApproachAngle2 = ApproachAngle2 };
     }
 }