feat: add Box.Translate and improve NFP/IFP geometry APIs

Add immutable Translate methods to Box. Make NoFitPolygon
ToClipperPath/FromClipperPath public with optional offset parameter.
Refactor InnerFitPolygon.ComputeFeasibleRegion to accept PathsD
directly, letting Clipper2 handle implicit union. Add UpdateBounds
calls after polygon construction.

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

OpenNest.Core/Geometry/Box.cs

@@ -74,6 +74,16 @@ namespace OpenNest.Geometry
             Location += voffset;
         }
 
+        public Box Translate(double x, double y)
+        {
+            return new Box(X + x, Y + y, Width, Length);
+        }
+
+        public Box Translate(Vector offset)
+        {
+            return new Box(X + offset.X, Y + offset.Y, Width, Length);
+        }
+
         public double Left
         {
             get { return X; }