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})";
+    }
+}

OpenNest.Tests/Cincinnati/CoordinateFormatterTests.cs

@@ -0,0 +1,40 @@
+using OpenNest.Posts.Cincinnati;
+
+namespace OpenNest.Tests.Cincinnati;
+
+public class CoordinateFormatterTests
+{
+    [Theory]
+    [InlineData(13.401, 4, "13.401")]
+    [InlineData(13.0, 4, "13")]
+    [InlineData(0.0, 4, "0")]
+    [InlineData(57.4895, 4, "57.4895")]
+    [InlineData(13.401, 3, "13.401")]
+    [InlineData(13.4016, 3, "13.402")]
+    public void FormatCoord_FormatsCorrectly(double value, int accuracy, string expected)
+    {
+        var formatter = new CoordinateFormatter(accuracy);
+        Assert.Equal(expected, formatter.FormatCoord(value));
+    }
+
+    [Theory]
+    [InlineData(-5.25, 4, "-5.25")]
+    [InlineData(-0.001, 4, "-0.001")]
+    public void FormatCoord_HandlesNegatives(double value, int accuracy, string expected)
+    {
+        var formatter = new CoordinateFormatter(accuracy);
+        Assert.Equal(expected, formatter.FormatCoord(value));
+    }
+
+    [Fact]
+    public void Comment_FormatsWithSpaces()
+    {
+        Assert.Equal("( hello )", CoordinateFormatter.Comment("hello"));
+    }
+
+    [Fact]
+    public void InlineComment_FormatsWithoutSpaces()
+    {
+        Assert.Equal("(hello)", CoordinateFormatter.InlineComment("hello"));
+    }
+}