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/PartSequencerFactory.cs

@@ -1,5 +1,5 @@
-using OpenNest.CNC.CuttingStrategy;
 using System;
+using OpenNest.CNC.CuttingStrategy;
 
 namespace OpenNest.Engine.Sequencing
 {
@@ -16,7 +16,8 @@ namespace OpenNest.Engine.Sequencing
                 SequenceMethod.LeastCode => new LeastCodeSequencer(),
                 SequenceMethod.Advanced => new AdvancedSequencer(parameters),
                 _ => throw new NotSupportedException(
-                    $"Sequence method '{parameters.Method}' is not supported.")
+                    $"Sequence method '{parameters.Method}' is not supported."
+                ),
             };
         }
     }