fix(engine): stop PairFiller toggling the global fill-strategy registry

PairFiller.EvaluateCandidates wrapped its candidate loop in
FillStrategyRegistry.SetEnabled(...) and reset it with SetEnabled(null)
afterwards. Nothing inside that window re-enters the strategy pipeline
(EvaluateCandidate and FillRemnantBox only use FillLinear), so the
restriction had no effect on the running solve. It did mutate process-global
state: concurrent solves (e.g. benchmark --parallel) could observe the
narrowed strategy list, and the reset cleared any filter set elsewhere.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

OpenNest.Engine/Fill/PairFiller.cs

@@ -97,7 +97,6 @@ namespace OpenNest.Engine.Fill
             var partBox = drawing.Program.BoundingBox();
             var partArea = System.Math.Max(partBox.Width * partBox.Length, 1);
 
-            FillStrategyRegistry.SetEnabled("Pairs", "RectBestFit", "Extents", "Linear");
             try
             {
                 for (var batchStart = 0; batchStart < candidates.Count; batchStart += batchSize)
@@ -165,10 +164,6 @@ namespace OpenNest.Engine.Fill
             {
                 Debug.WriteLine("[PairFiller] Cancelled mid-phase, using results so far");
             }
-            finally
-            {
-                FillStrategyRegistry.SetEnabled(null);
-            }
 
             Debug.WriteLine($"[PairFiller] Best pair result: {best?.Count ?? 0} parts");
             return best ?? new List<Part>();