feat: add CoordinateFormatter for Cincinnati G-code coordinate output

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

OpenNest.Posts.Cincinnati/CoordinateFormatter.cs

@@ -0,0 +1,23 @@
+namespace OpenNest.Posts.Cincinnati
+{
+    public sealed class CoordinateFormatter
+    {
+        private readonly int _accuracy;
+        private readonly string _format;
+
+        public CoordinateFormatter(int accuracy)
+        {
+            _accuracy = accuracy;
+            _format = "0." + new string('#', accuracy);
+        }
+
+        public string FormatCoord(double value)
+        {
+            return System.Math.Round(value, _accuracy).ToString(_format);
+        }
+
+        public static string Comment(string text) => $"( {text} )";
+
+        public static string InlineComment(string text) => $"({text})";
+    }
+}