refactor(fill): extract constants and EvaluateCandidate in PairFiller

Extract magic numbers into named constants (MaxTopCandidates,
EarlyExitMinTried, etc.), extract candidate evaluation into
EvaluateCandidate method, and expose BestFits property so
PairsFillStrategy can reuse without redundant BestFitCache call.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest.Engine/Strategies/PairsFillStrategy.cs

@@ -1,4 +1,3 @@
-using OpenNest.Engine.BestFit;
 using OpenNest.Engine.Fill;
 using System.Collections.Generic;
 
@@ -16,11 +15,7 @@ namespace OpenNest.Engine.Strategies
             var result = filler.Fill(context.Item, context.WorkArea,
                 context.PlateNumber, context.Token, context.Progress);
 
-            // Cache hit — PairFiller already called GetOrCompute internally.
-            var bestFits = BestFitCache.GetOrCompute(
-                context.Item.Drawing, context.Plate.Size.Length,
-                context.Plate.Size.Width, context.Plate.PartSpacing);
-            context.SharedState["BestFits"] = bestFits;
+            context.SharedState["BestFits"] = filler.BestFits;
 
             return result;
         }