perf(geometry): flatten Shape.ToPolygon by chord tolerance by default

ToPolygon() used 1000 segments for every arc and circle regardless of
size: no more accurate for large arcs and needlessly dense for small
ones, which made every Drawing area and canonical-angle computation on
an arc-heavy part handle hundreds of thousands of vertices.

The parameterless overload now delegates to ToPolygonWithTolerance with
DefaultChordTolerance = 0.0001. That is ten times tighter than the
spacing and overlap checks' 0.001 because callers use this polygon for
area, which an inscribed polygon always under-reads: at 0.001, seven
area-sensitive cut-off, splitter and circle-packing tests drifted past
their precision; at 0.0001 all pass. ToPolygon(int) keeps the fixed
segment count for any caller that wants it.

With the one-pass Cleanup, a 300-nest PEP archive import on the forge
server went from 46.1 s to 15.5 s.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

OpenNest.Core/Geometry/Shape.cs

@@ -264,10 +264,24 @@ namespace OpenNest.Geometry
         }
 
         /// <summary>
-        /// Converts the shape to a polygon.
+        /// Maximum chord error, in drawing units, when <see cref="ToPolygon()"/> flattens arcs
+        /// and circles. Ten times tighter than the spacing/overlap checks' 0.001 because callers
+        /// use this polygon for area, and an inscribed polygon always reads low.
         /// </summary>
-        /// <returns></returns>
-        public Polygon ToPolygon(int arcSegments = 1000)
+        public const double DefaultChordTolerance = 0.0001;
+
+        /// <summary>
+        /// Converts the shape to a polygon, flattening arcs to within
+        /// <see cref="DefaultChordTolerance"/>. A fixed segment count per arc (formerly 1000)
+        /// made large arcs no more accurate and small ones needlessly dense: arc-heavy drawings
+        /// produced hundreds of thousands of vertices every time their area was computed.
+        /// </summary>
+        public Polygon ToPolygon() => ToPolygonWithTolerance(DefaultChordTolerance);
+
+        /// <summary>
+        /// Converts the shape to a polygon with a fixed number of segments per arc and circle.
+        /// </summary>
+        public Polygon ToPolygon(int arcSegments)
         {
             var polygon = new Polygon();
 

OpenNest.Core/Geometry/SpacingOutline.cs

@@ -64,8 +64,8 @@ namespace OpenNest.Geometry
         public static SpacingOutline Build(ShapeProfile profile, Vector location, double inflateBy)
         {
             ArgumentNullException.ThrowIfNull(profile);
-            // Adaptive tolerance instead of Shape.ToPolygon()'s default (up to 1000 segments per
-            // arc): arc-heavy real parts otherwise produce thousands of vertices.
+            // Flatten by chord tolerance, not a fixed segment count: arc-heavy real parts
+            // otherwise produce thousands of vertices.
             var region = ClipperBridge.OffsetForValidation(
                 profile,
                 inflateBy > Tolerance.Epsilon ? inflateBy : 0,