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/BestFit/BestFitOverlapTests.cs

@@ -58,9 +58,11 @@ public class BestFitOverlapTests
             if (parts[0].Intersects(parts[1], out var pts))
             {
                 overlapping++;
-                _output.WriteLine($"  OVERLAP #{overlapping}: Test {result.Candidate.TestNumber} " +
-                    $"Part2Rot={OpenNest.Math.Angle.ToDegrees(result.Candidate.Part2Rotation):F1}° " +
-                    $"collision pts={pts.Count}");
+                _output.WriteLine(
+                    $"  OVERLAP #{overlapping}: Test {result.Candidate.TestNumber} "
+                        + $"Part2Rot={OpenNest.Math.Angle.ToDegrees(result.Candidate.Part2Rotation):F1}° "
+                        + $"collision pts={pts.Count}"
+                );
             }
         }