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/Splitting/EntitySplitTests.cs

@@ -98,7 +98,8 @@ public class EntitySplitTests
         var expectedHigh = 50.0 + System.Math.Sqrt(300);
         Assert.True(
             System.Math.Abs(y - expectedLow) < 0.1 || System.Math.Abs(y - expectedHigh) < 0.1,
-            $"Expected Y near {expectedLow:F2} or {expectedHigh:F2}, got {y:F2}");
+            $"Expected Y near {expectedLow:F2} or {expectedHigh:F2}, got {y:F2}"
+        );
     }
 
     // --- CrossesSplitLine ---