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/Sequence.cs

@@ -1,5 +1,5 @@
-using OpenNest.Geometry;
-using System.Collections.Generic;
+using System.Collections.Generic;
+using OpenNest.Geometry;
 
 namespace OpenNest
 {
@@ -28,7 +28,14 @@ namespace OpenNest
             seqList.Add(lastPart);
             dupList.Remove(lastPart);
 
-            for (int i = 0; i < parts.Count - 1 /*STOP BEFORE LAST PART*/; i++)
+            for (
+                int i = 0;
+                i
+                    < parts.Count
+                        - 1 /*STOP BEFORE LAST PART*/
+                ;
+                i++
+            )
             {
                 var nextPart = GetClosestPart(lastPart.Location, dupList);