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.Api/NestRequestPlate.cs

@@ -7,6 +7,7 @@ public class NestRequestPlate
 {
     public string Id { get; init; }
     public Size Size { get; init; }
+
     /// <summary>Available physical sheets; null means unlimited.</summary>
     public int? Quantity { get; init; }
     public double PartSpacing { get; init; }