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/CNC/VariableDefinition.cs

@@ -8,8 +8,13 @@ namespace OpenNest.CNC
         public bool Inline { get; }
         public bool Global { get; }
 
-        public VariableDefinition(string name, string expression, double value,
-            bool inline = false, bool global = false)
+        public VariableDefinition(
+            string name,
+            string expression,
+            double value,
+            bool inline = false,
+            bool global = false
+        )
         {
             Name = name;
             Expression = expression;