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/TShapeTests.cs

@@ -30,7 +30,13 @@ public class TShapeTests
     [Fact]
     public void GetDrawing_CustomStemAndBarDimensions()
     {
-        var shape = new TShape { Width = 12, Height = 18, StemWidth = 6, BarHeight = 4 };
+        var shape = new TShape
+        {
+            Width = 12,
+            Height = 18,
+            StemWidth = 6,
+            BarHeight = 4,
+        };
         var drawing = shape.GetDrawing();
 
         // Area = Width * BarHeight + StemWidth * (Height - BarHeight)