Multi-Body Grid: Visually delegate native vertical scrollbar UX with CSS

When lock: end columns exist in a multi-body grid, the native vertical scrollbar visually appears between the center rows and the locked end rows. This breaks UX expectations.

Instead of reverting to a custom and slow VerticalScrollbar component, we will implement a CSS-driven visual delegation pattern:

1. Apply a .neo-hide-scrollbar SCSS class to the bodyStart and body grid wrappers so their native overflow-y capability remains functional for mouse wheels/trackpads, but is hidden from the UI.
2. The lock: end wrapper (or center if lock: end is absent) will remain the only visually rendered trackway.
3. Update GridDragScroll to gracefully ignore hidden scrollbars when calculating thumb-drag boundaries.
4. Repoint the GridRowScrollPinning Addon listener logic to attach dynamically to whichever wrapper natively holds the active scrollbar thumb.