style: apply CSharpier formatting to all C# sources

Repo-wide sweep with the pinned CSharpier 1.3.0 tool. Whitespace and
line-wrapping only; OpenNest.Engine.Tests (109) and OpenNest.IO.Tests
pass after reformat, full solution builds 0 errors.

Added .csharpierignore so csproj/config XML keeps its existing layout
(CSharpier's XML wrapping churns attributes with zero benefit).

Formatting is now enforceable: dotnet csharpier check . passes.

OpenNest.Tests/BestFit/NfpBestFitIntegrationTests.cs

@@ -38,9 +38,7 @@ public class NfpBestFitIntegrationTests
         var drawing = TestHelpers.MakeLShapeDrawing();
         var results = finder.FindBestFits(drawing);
 
-        var bestUtilization = results
-            .Where(r => r.Keep)
-            .Max(r => r.Utilization);
+        var bestUtilization = results.Where(r => r.Keep).Max(r => r.Utilization);
         Assert.True(bestUtilization > 0.5);
     }
 
@@ -51,7 +49,6 @@ public class NfpBestFitIntegrationTests
         var drawing = TestHelpers.MakeSquareDrawing();
         var results = finder.FindBestFits(drawing);
 
-        Assert.All(results.Where(r => r.Keep), r =>
-            Assert.Equal("Valid", r.Reason));
+        Assert.All(results.Where(r => r.Keep), r => Assert.Equal("Valid", r.Reason));
     }
 }