feat: add IConfigurablePostProcessor interface and implement in Cincinnati post

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

OpenNest.Tests/Cincinnati/CincinnatiPostProcessorTests.cs

@@ -121,6 +121,17 @@ public class CincinnatiPostProcessorTests
         Assert.Equal("OpenNest", pp.Author);
     }
 
+    [Fact]
+    public void Post_ImplementsIConfigurablePostProcessor()
+    {
+        var post = new CincinnatiPostProcessor(new CincinnatiPostConfig());
+        var configurable = post as IConfigurablePostProcessor;
+
+        Assert.NotNull(configurable);
+        Assert.NotNull(configurable.Config);
+        Assert.IsType<CincinnatiPostConfig>(configurable.Config);
+    }
+
     [Fact]
     public void Post_SkipsEmptyPlates()
     {