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.