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/NfpSlideStrategy.cs

@@ -22,14 +22,14 @@ namespace OpenNest.Engine.BestFit
             Polygon stationaryPerimeter, Polygon stationaryHull, Vector correction)
         {
             _part2Rotation = part2Rotation;
-            Type = type;
+            StrategyIndex = type;
             Description = description;
             _stationaryPerimeter = stationaryPerimeter;
             _stationaryHull = stationaryHull;
             _correction = correction;
         }
 
-        public int Type { get; }
+        public int StrategyIndex { get; }
         public string Description { get; }
 
         /// <summary>
@@ -155,7 +155,7 @@ namespace OpenNest.Engine.BestFit
                 Part1Rotation = 0,
                 Part2Rotation = _part2Rotation,
                 Part2Offset = offset,
-                StrategyType = Type,
+                StrategyIndex = StrategyIndex,
                 TestNumber = testNumber,
                 Spacing = spacing
             };