fix: correct NFP polygon computation and inflation direction

Three bugs fixed in NfpSlideStrategy pipeline:

1. NoFitPolygon.Reflect() incorrectly reversed vertex order. Point
   reflection (negating both axes) is a 180° rotation that preserves
   winding — the Reverse() call was converting CCW to CW, producing
   self-intersecting bowtie NFPs.

2. PolygonHelper inflation used OffsetSide.Left which is inward for
   CCW perimeters. Changed to OffsetSide.Right for outward inflation
   so NFP boundary positions give properly-spaced part placements.

3. Removed incorrect correction vector — same-drawing pairs have
   identical polygon-to-part offsets that cancel out in the NFP
   displacement.

Also refactored NfpSlideStrategy to be immutable (removed mutable
cache fields, single constructor with required data, added Create
factory method). BestFitFinder remains on RotationSlideStrategy
as default.

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

OpenNest.Core/Geometry/NoFitPolygon.cs

@@ -1,4 +1,5 @@
 using Clipper2Lib;
+using OpenNest.Math;
 using System.Collections.Generic;
 
 namespace OpenNest.Geometry
@@ -22,8 +23,20 @@ namespace OpenNest.Geometry
             return MinkowskiSum(stationary, reflected);
         }
 
+        /// <summary>
+        /// Optimized version of Compute for polygons known to be convex.
+        /// Bypasses expensive triangulation and Clipper unions.
+        /// </summary>
+        public static Polygon ComputeConvex(Polygon stationary, Polygon orbiting)
+        {
+            var reflected = Reflect(orbiting);
+            return ConvexMinkowskiSum(stationary, reflected);
+        }
+
         /// <summary>
         /// Reflects a polygon through the origin (negates all vertex coordinates).
+        /// Point reflection (negating both axes) is equivalent to 180° rotation,
+        /// which preserves winding order. No reversal needed.
         /// </summary>
         private static Polygon Reflect(Polygon polygon)
         {
@@ -32,8 +45,6 @@ namespace OpenNest.Geometry
             foreach (var v in polygon.Vertices)
                 result.Vertices.Add(new Vector(-v.X, -v.Y));
 
-            // Reflecting reverses winding order — reverse to maintain CCW.
-            result.Vertices.Reverse();
             return result;
         }
 
@@ -83,14 +94,19 @@ namespace OpenNest.Geometry
             var edgesA = GetEdgeVectors(a);
             var edgesB = GetEdgeVectors(b);
 
-            // Find bottom-most (then left-most) vertex for each polygon as starting point.
+            // Find indices of bottom-left vertices for both.
             var startA = FindBottomLeft(a);
             var startB = FindBottomLeft(b);
 
             var result = new Polygon();
+            
+            // The starting point of the Minkowski sum A + B is the sum of the 
+            // starting points of A and B. For NFP = A + (-B), this is 
+            // startA + startReflectedB.
             var current = new Vector(
                 a.Vertices[startA].X + b.Vertices[startB].X,
                 a.Vertices[startA].Y + b.Vertices[startB].Y);
+            
             result.Vertices.Add(current);
 
             var ia = 0;
@@ -98,7 +114,6 @@ namespace OpenNest.Geometry
             var na = edgesA.Count;
             var nb = edgesB.Count;
 
-            // Reorder edges to start from the bottom-left vertex.
             var orderedA = ReorderEdges(edgesA, startA);
             var orderedB = ReorderEdges(edgesB, startB);
 
@@ -117,7 +132,10 @@ namespace OpenNest.Geometry
                 else
                 {
                     var angleA = System.Math.Atan2(orderedA[ia].Y, orderedA[ia].X);
+                    if (angleA < 0) angleA += Angle.TwoPI;
+
                     var angleB = System.Math.Atan2(orderedB[ib].Y, orderedB[ib].X);
+                    if (angleB < 0) angleB += Angle.TwoPI;
 
                     if (angleA < angleB)
                     {
@@ -129,7 +147,6 @@ namespace OpenNest.Geometry
                     }
                     else
                     {
-                        // Same angle — merge both edges.
                         edge = new Vector(
                             orderedA[ia].X + orderedB[ib].X,
                             orderedA[ia].Y + orderedB[ib].Y);
@@ -143,6 +160,7 @@ namespace OpenNest.Geometry
             }
 
             result.Close();
+            result.UpdateBounds();
             return result;
         }