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/IO/SubProgramSerializationTests.cs

@@ -54,7 +54,14 @@ public class SubProgramSerializationTests
 
         var pgm = new Program(Mode.Absolute);
         pgm.SubPrograms[42] = sub;
-        pgm.Codes.Add(new SubProgramCall { Id = 42, Program = sub, Offset = new Vector(5, 5) });
+        pgm.Codes.Add(
+            new SubProgramCall
+            {
+                Id = 42,
+                Program = sub,
+                Offset = new Vector(5, 5),
+            }
+        );
         // Add perimeter so the drawing has non-zero geometry
         pgm.Codes.Add(new RapidMove(0, 0));
         pgm.Codes.Add(new LinearMove(10, 0));