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/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} )";