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/Layer.cs

@@ -7,7 +7,7 @@ namespace OpenNest.Geometry
         public static readonly Layer Default = new Layer("0")
         {
             Color = Color.White,
-            IsVisible = true
+            IsVisible = true,
         };
 
         public Layer(string name)