Move geometry primitives to OpenNest.Geometry namespace

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

OpenNest.Core/Geometry/Spacing.cs

@@ -0,0 +1,27 @@
+namespace OpenNest.Geometry
+{
+    public struct Spacing
+    {
+        public Spacing(double topBottom, double leftRight)
+        {
+            Top = Bottom = topBottom;
+            Left = Right = leftRight;
+        }
+
+        public Spacing(double left, double bottom, double right, double top)
+        {
+            Left = left;
+            Bottom = bottom;
+            Right = right;
+            Top = top;
+        }
+
+        public double Left;
+
+        public double Bottom;
+
+        public double Right;
+
+        public double Top;
+    }
+}