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/Geometry/Size.cs

@@ -44,6 +44,7 @@ namespace OpenNest.Geometry
 
         public override string ToString() => $"{Width} x {Length}";
 
-        public string ToString(int decimalPlaces) => $"{System.Math.Round(Width, decimalPlaces)} x {System.Math.Round(Length, decimalPlaces)}";
+        public string ToString(int decimalPlaces) =>
+            $"{System.Math.Round(Width, decimalPlaces)} x {System.Math.Round(Length, decimalPlaces)}";
     }
 }