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/RelativePosition.cs

@@ -1,5 +1,4 @@
-
-namespace OpenNest
+namespace OpenNest
 {
     public enum RelativePosition
     {
@@ -8,6 +7,6 @@ namespace OpenNest
         Right,
         Top,
         Bottom,
-        None
+        None,
     }
 }