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/ProgramVariableManager.cs

@@ -20,8 +20,8 @@ namespace OpenNest.CNC
 
         public List<string> EmitDeclarations()
         {
-            return _variables.Values
-                .Where(v => v.Expression != null)
+            return _variables
+                .Values.Where(v => v.Expression != null)
                 .OrderBy(v => v.Number)
                 .Select(v => $"{v.Reference}={v.Expression} ({FormatComment(v.Name)})")
                 .ToList();