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.Core/Splitting/ISplitFeature.cs

@@ -18,5 +18,10 @@ public class SplitFeatureResult
 public interface ISplitFeature
 {
     string Name { get; }
-    SplitFeatureResult GenerateFeatures(SplitLine line, double extentStart, double extentEnd, SplitParameters parameters);
+    SplitFeatureResult GenerateFeatures(
+        SplitLine line,
+        double extentStart,
+        double extentEnd,
+        SplitParameters parameters
+    );
 }