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/Jobs/INestingEngine.cs

@@ -6,5 +6,9 @@ namespace OpenNest;
 /// <summary>Synchronous whole-job solver. Cancellation throws, rather than returning partial success.</summary>
 public interface INestingEngine
 {
-    NestJobResult Solve(NestJob job, IProgress<NestJobProgress> progress = null, CancellationToken token = default);
+    NestJobResult Solve(
+        NestJob job,
+        IProgress<NestJobProgress> progress = null,
+        CancellationToken token = default
+    );
 }