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/Strategies/ColumnFillStrategy.cs

@@ -14,7 +14,10 @@ public class ColumnFillStrategy : IFillStrategy
         if (context.PartType == PartType.Rectangle)
             return null;
 
-        var filler = new StripeFiller(context, NestDirection.Vertical) { CompleteStripesOnly = true };
+        var filler = new StripeFiller(context, NestDirection.Vertical)
+        {
+            CompleteStripesOnly = true,
+        };
         return filler.Fill();
     }
 }