refactor: merge duplicated geometry, settings-file and form helpers

Roslyn Bridge flagged these as copy-paste clusters; each pair now shares
one implementation so a fix lands everywhere at once:

- Shape.OffsetInward/OffsetOutward differed only in the target winding.
- BestFitFinder kept its own copy of SpatialQuery's arc-extreme points.
- SplitDrawingForm rebuilt DrawingSplitter's clip-region grid for preview.
- GeometryOptimizer's collinear/coradial finders (one misspelled
  GetCoradialArs) become one generic FindRelated.
- Five settings files repeated the write-with-retry loop, and two also
  repeated the unit parser and unreadable-file filter (SettingsFile).
- ClampsForm and SupportGridForm repeated their row/box builders.
- Vertical/HorizontalRemnantComparer share a RemnantComparer base.

Behavior is unchanged except that Engine/NestStorage settings loads now
also tolerate NotSupported/Security exceptions, matching the other
settings files' never-throw contract.

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

OpenNest.Core/Geometry/SpatialQuery.cs

@@ -791,7 +791,11 @@ namespace OpenNest.Geometry
             return vertices.ToArray();
         }
 
-        private static void AddArcExtremeVertices(HashSet<Vector> points, Arc arc)
+        /// <summary>
+        /// Adds the cardinal extreme points of an arc (0°, 90°, 180°, 270°)
+        /// if they fall within the arc's angular span.
+        /// </summary>
+        internal static void AddArcExtremeVertices(ICollection<Vector> points, Arc arc)
         {
             var a1 = arc.StartAngle;
             var a2 = arc.EndAngle;