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.Posts.Cincinnati/SpeedClassifier.cs

@@ -8,8 +8,10 @@ namespace OpenNest.Posts.Cincinnati
         public string Classify(double contourLength, double sheetDiagonal)
         {
             var ratio = contourLength / sheetDiagonal;
-            if (ratio >= FastThreshold) return "FAST";
-            if (ratio <= SlowThreshold) return "SLOW";
+            if (ratio >= FastThreshold)
+                return "FAST";
+            if (ratio <= SlowThreshold)
+                return "SLOW";
             return "MEDIUM";
         }