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.Core/Geometry/Vector.cs

@@ -1,5 +1,5 @@
-using OpenNest.Math;
-using System;
+using System;
+using OpenNest.Math;
 
 namespace OpenNest.Geometry
 {
@@ -31,7 +31,7 @@ namespace OpenNest.Geometry
         {
             unchecked
             {
-                // Use a simple but effective hash combine. 
+                // Use a simple but effective hash combine.
                 // We use a small epsilon-safe rounding if needed, but for uniqueness in HashSet
                 // during a single operation, raw bits or slightly rounded is usually fine.
                 // However, IsEqualTo uses Tolerance.Epsilon, so we should probably round to some precision.