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/PushDirection.cs

@@ -1,4 +1,3 @@
-
 namespace OpenNest
 {
     public enum PushDirection
@@ -6,6 +5,6 @@ namespace OpenNest
         Up,
         Down,
         Left,
-        Right
+        Right,
     }
 }