fix: enable PositionStrategy and DragConfig.threshold extension points (#2217) (#2217)

Two extension points were defined in the v2 API but never actually used:

1. PositionStrategy.calcStyle() and calcDragPosition() methods were never
   called - only type and scale properties were extracted. Now custom
   position strategies can override positioning and drag position calculation.

2. DragConfig.threshold was defined with a default of 3px but never passed
   to GridItem. Now the threshold is properly implemented - drag callbacks
   won't fire until the mouse moves the threshold distance from the initial
   mousedown position.

For backwards compatibility:
- Legacy API sets threshold to 0 (immediate drag callbacks)
- v2 API defaults to threshold of 3px (fixes #1341, #1401)