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/Api/CutParametersTests.cs

@@ -25,7 +25,7 @@ public class CutParametersTests
             PierceTime = TimeSpan.FromSeconds(1.0),
             LeadInLength = 0.25,
             PostProcessor = "CL-707",
-            Units = Units.Millimeters
+            Units = Units.Millimeters,
         };
 
         Assert.Equal(200, cp.Feedrate);