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/RectanglePacking/FillSameRotation.cs

@@ -5,9 +5,7 @@ namespace OpenNest.RectanglePacking
     internal class FillSameRotation : FillEngine
     {
         public FillSameRotation(Bin bin)
-            : base(bin)
-        {
-        }
+            : base(bin) { }
 
         public override void Fill(Item item)
         {