feat: add snap-to-endpoint/midpoint for lead-in placement

Priority-based snapping: when the cursor is within 10px of an entity
endpoint or midpoint, snaps to it instead of the nearest contour point.
Diamond marker (endpoint) or triangle marker (midpoint) replaces the
lime dot to indicate active snap. Also refactors OnPaint into focused
helper methods and adds Arc.MidPoint().

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

OpenNest.Core/Geometry/Arc.cs

@@ -155,6 +155,17 @@ namespace OpenNest.Geometry
                 Center.Y + Radius * System.Math.Sin(EndAngle));
         }
 
+        /// <summary>
+        /// Mid point of the arc (point at the angle midway between start and end).
+        /// </summary>
+        public Vector MidPoint()
+        {
+            var midAngle = StartAngle + (IsReversed ? -SweepAngle() / 2 : SweepAngle() / 2);
+            return new Vector(
+                Center.X + Radius * System.Math.Cos(midAngle),
+                Center.Y + Radius * System.Math.Sin(midAngle));
+        }
+
         /// <summary>
         /// Splits the arc at the given point, returning two sub-arcs.
         /// Either half may be null if the split point coincides with an endpoint.