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.Tests/BestFit/NfpSlideStrategyTests.cs

@@ -119,6 +119,9 @@ public class NfpSlideStrategyTests
                 validCount++;
         }
 
-        Assert.True(validCount > 0, $"No non-overlapping candidates found out of {candidates.Count} total. Candidate 0 offset: {candidates[0].Part2Offset}");
+        Assert.True(
+            validCount > 0,
+            $"No non-overlapping candidates found out of {candidates.Count} total. Candidate 0 offset: {candidates[0].Part2Offset}"
+        );
     }
 }