refactor: extract IDistanceComputer with CPU and GPU implementations

Extract distance computation from RotationSlideStrategy into a pluggable
IDistanceComputer interface. CpuDistanceComputer adds leading-face vertex
culling (~50% fewer rays per direction) with early exit on overlap.
GpuDistanceComputer wraps ISlideComputer with Line-to-flat-array conversion.
SlideOffset struct uses direction vectors (DirX/DirY) instead of PushDirection.
SpatialQuery.RayEdgeDistance(dirX,dirY) made public for CPU path.

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

OpenNest.Engine/BestFit/SlideOffset.cs

@@ -0,0 +1,18 @@
+namespace OpenNest.Engine.BestFit
+{
+    public readonly struct SlideOffset
+    {
+        public double Dx { get; }
+        public double Dy { get; }
+        public double DirX { get; }
+        public double DirY { get; }
+
+        public SlideOffset(double dx, double dy, double dirX, double dirY)
+        {
+            Dx = dx;
+            Dy = dy;
+            DirX = dirX;
+            DirY = dirY;
+        }
+    }
+}