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.Engine/NestDirection.cs

@@ -1,9 +1,8 @@
-
-namespace OpenNest
+namespace OpenNest
 {
     public enum NestDirection
     {
         Vertical,
-        Horizontal
+        Horizontal,
     }
 }