Grid: Implement Mathematical Column Layout Engine

[tobiu]

This is a sub-task of Epic #9456 (Buffered Grid - High-Performance Locked Columns).

Context:
Currently, grid.header.Toolbar#passSizeToBody calculates column x positions by calling getDomRect on the header items.
This poses a problem for locked columns using CSS transforms (transform: translateX). getDomRect will return the visually transformed layout coordinates, breaking the logical representation of the Grid's layout state.

Task:
Refactor the column layout calculation in GridContainer / HeaderToolbar so that columnPositions.x is calculated mathematically.

• Logical X should equal the sum of the widths (including borders/margins) of all preceding columns in the items array.
• This entirely decouples logical x layout from visual CSS states and avoids the need for asynchronous getDomRect queries on individual header buttons, improving startup performance.

Note: The Container width and Toolbar height/width still need to be measured once for container sizing.