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/IPlateNester.cs

@@ -6,6 +6,9 @@ namespace OpenNest;
 /// <summary>Places on one sheet only. Must not change stock, demand, or caller-owned domain objects.</summary>
 public interface IPlateNester
 {
-    PlateCandidate Place(PlatePlacementRequest request, IProgress<NestJobProgress> progress = null,
-        CancellationToken token = default);
+    PlateCandidate Place(
+        PlatePlacementRequest request,
+        IProgress<NestJobProgress> progress = null,
+        CancellationToken token = default
+    );
 }