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/Fill/FillResultCache.cs

@@ -76,9 +76,10 @@ public static class FillResultCache
         }
 
         public bool Equals(CacheKey other) =>
-            ReferenceEquals(Drawing, other.Drawing) &&
-            Width == other.Width && Height == other.Height &&
-            Spacing == other.Spacing;
+            ReferenceEquals(Drawing, other.Drawing)
+            && Width == other.Width
+            && Height == other.Height
+            && Spacing == other.Spacing;
 
         public override bool Equals(object obj) => obj is CacheKey other && Equals(other);