perf(grid): avoid setState on every horizontal scroll tick

[ZhuchkaTriplesix]

Summary

VirtualResultGrid._onHorizontalScroll setStates _scrollOffset whenever the offset moves ≥0.5px. Wide grids then rebuild the virtualized body on every pan frame at 120 Hz. Header/body sync does not need a full setState if offset is applied via a ListenableBuilder / NotificationListener on the scrolling layer only.

Scope

• Drive sticky header / column clip from the scroll controller without rebuilding row items.
• Keep the 0.5px dead zone or replace with a transform.

Out of scope

• Column auto-fit sample scan in LayoutBuilder.