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.Tests/Shapes/RectangleShapeTests.cs

@@ -27,7 +27,12 @@ public class RectangleShapeTests
     [Fact]
     public void GetDrawing_CustomName_IsUsed()
     {
-        var shape = new RectangleShape { Name = "Plate1", Length = 10, Width = 5 };
+        var shape = new RectangleShape
+        {
+            Name = "Plate1",
+            Length = 10,
+            Width = 5,
+        };
         var drawing = shape.GetDrawing();
 
         Assert.Equal("Plate1", drawing.Name);