Grid Multi-Body: Decoupled Horizontal Scroller & Main Thread Sync

Phase 3 of the Multi-Body Epic (#9486).

To guarantee zero vertical jitter, the V2 architecture places the Center SubGrid inside an outer wrapper that handles vertical scrolling. This pushes the native horizontal scrollbar of the Center SubGrid to the absolute bottom of the virtualized data (e.g., 500,000px down), making it unusable.

We must build a decoupled, "fake" horizontal scrollbar that stays pinned to the bottom of the visible grid viewport.

Requirements:

1. Neo.grid.HorizontalScroller Component:
   • Create a new component that renders a native horizontal scrollbar but contains no actual data.
   • DOM structure concept: <div style="overflow-x: auto"><div style="width: {TotalCenterColumnsWidth}px"></div></div>
   • Position it as a sibling to the grid.Container body wrapper (e.g., just above the Footer Toolbar).
2. Main Thread Addon (GridHorizontalScrollSync):
   • Create a new Main Thread Addon to handle the high-frequency syncing.
   • It must register the DOM node of the HorizontalScroller and the DOM nodes of the Center SubGrid and Center Header.
   • It must attach a scroll listener to the fake scroller.
   • On scroll, it must synchronously execute centerSubGridNode.scrollLeft = scrollLeft and centerHeaderNode.scrollLeft = scrollLeft.
3. Visibility Logic:
   • The HorizontalScroller should only render/be visible if the Center SubGrid actually requires horizontal scrolling (i.e., total center column width > available center viewport width).