refactor: use IDistanceComputer and rename Type to StrategyIndex

Wire IDistanceComputer into RotationSlideStrategy, replacing inline
CPU/GPU branching. BestFitFinder constructs the appropriate implementation.
Replace PushDirection enum with direction vectors in BuildOffsets.
Rename IBestFitStrategy.Type and PairCandidate.StrategyType to StrategyIndex
for clarity (JSON field name unchanged for backward compatibility).

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

OpenNest.Engine/BestFit/PairCandidate.cs

@@ -8,7 +8,7 @@ namespace OpenNest.Engine.BestFit
         public double Part1Rotation { get; set; }
         public double Part2Rotation { get; set; }
         public Vector Part2Offset { get; set; }
-        public int StrategyType { get; set; }
+        public int StrategyIndex { get; set; }
         public int TestNumber { get; set; }
         public double Spacing { get; set; }
     }