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/Sequencing/PlateHelper.cs

@@ -15,7 +15,7 @@ namespace OpenNest.Engine.Sequencing
                 2 => new Vector(0, yExtent),
                 3 => new Vector(0, 0),
                 4 => new Vector(xExtent, 0),
-                _ => new Vector(xExtent, yExtent)
+                _ => new Vector(xExtent, yExtent),
             };
         }
     }