fix: review fixes — culture-invariant formatting, sealed config, threshold boundary

- Use CultureInfo.InvariantCulture in CoordinateFormatter, SpeedClassifier,
  and CincinnatiPreambleWriter to prevent locale-dependent G-code output
- Make CincinnatiPostConfig sealed per spec
- Fix SpeedClassifier.Classify threshold to >= (matching spec)

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

OpenNest.Posts.Cincinnati/CincinnatiPostConfig.cs

@@ -88,7 +88,7 @@ namespace OpenNest.Posts.Cincinnati
     /// Configuration for Cincinnati post processor.
     /// Defines machine-specific parameters, output format, and cutting strategies.
     /// </summary>
-    public class CincinnatiPostConfig
+    public sealed class CincinnatiPostConfig
     {
         /// <summary>
         /// Gets or sets the configuration name/identifier.

OpenNest.Posts.Cincinnati/CincinnatiPreambleWriter.cs

@@ -24,7 +24,7 @@ public sealed class CincinnatiPreambleWriter
     {
         w.WriteLine(CoordinateFormatter.Comment($"NEST {nestName}"));
         w.WriteLine(CoordinateFormatter.Comment($"CONFIGURATION - {_config.ConfigurationName}"));
-        w.WriteLine(CoordinateFormatter.Comment(DateTime.Now.ToString("MM-dd-yyyy hh:mm:ss tt")));
+        w.WriteLine(CoordinateFormatter.Comment(DateTime.Now.ToString("MM-dd-yyyy hh:mm:ss tt", System.Globalization.CultureInfo.InvariantCulture)));
 
         if (!string.IsNullOrEmpty(materialDescription))
             w.WriteLine(CoordinateFormatter.Comment($"Material = {materialDescription}"));

OpenNest.Posts.Cincinnati/CoordinateFormatter.cs

@@ -13,7 +13,8 @@ namespace OpenNest.Posts.Cincinnati
 
         public string FormatCoord(double value)
         {
-            return System.Math.Round(value, _accuracy).ToString(_format);
+            return System.Math.Round(value, _accuracy)
+                .ToString(_format, System.Globalization.CultureInfo.InvariantCulture);
         }
 
         public static string Comment(string text) => $"( {text} )";

OpenNest.Posts.Cincinnati/SpeedClassifier.cs

@@ -8,7 +8,7 @@ namespace OpenNest.Posts.Cincinnati
         public string Classify(double contourLength, double sheetDiagonal)
         {
             var ratio = contourLength / sheetDiagonal;
-            if (ratio > FastThreshold) return "FAST";
+            if (ratio >= FastThreshold) return "FAST";
             if (ratio <= SlowThreshold) return "SLOW";
             return "MEDIUM";
         }
@@ -22,7 +22,7 @@ namespace OpenNest.Posts.Cincinnati
         {
             // Cincinnati convention: no leading zero for values < 1 (e.g., ".8702" not "0.8702")
             var rounded = System.Math.Round(value, 4);
-            var str = rounded.ToString("0.####");
+            var str = rounded.ToString("0.####", System.Globalization.CultureInfo.InvariantCulture);
             if (rounded > 0 && rounded < 1 && str.StartsWith("0."))
                 return str.Substring(1);
             return str;

OpenNest.Tests/Cincinnati/SpeedClassifierTests.cs

@@ -6,7 +6,8 @@ public class SpeedClassifierTests
 {
     [Theory]
     [InlineData(20.0, 10.0, "FAST")]
-    [InlineData(5.0, 10.0, "MEDIUM")]
+    [InlineData(5.0, 10.0, "FAST")]
+    [InlineData(4.9, 10.0, "MEDIUM")]
     [InlineData(0.5, 10.0, "SLOW")]
     public void Classify_ReturnsExpectedClass(double contourLength, double sheetDiagonal, string expected)
     {