Grid Multi-Body: Refactor Layout Engine & SubGrid Partitioning

Phase 2 of the Multi-Body Epic (#9486).

Before we can render locked columns smoothly, we must physically partition the Grid's layout engine. Currently, Neo.grid.Container passes a single, flat columnPositions array to a singleton Neo.grid.Body.

Requirements:

1. Column Partitioning: Refactor the math engine inside grid.header.Toolbar#passSizeToBody() (or wherever the logical sizes are calculated). Instead of generating one flat array of physical x coordinates and widths, it must group them by their locked state:

• startColumns: Array of {dataField, width, x} for columns where locked: 'start'. The x coordinates here start at 0.
• centerColumns: Array of {dataField, width, x} for unlocked columns. The x coordinates here also start at 0 (because they will live in their own physical container).
• endColumns: Array of {dataField, width, x} for columns where locked: 'end'. x starts at 0.

2. SubGrid Instantiation (The Wrapper):

• The grid.Container needs a new internal structural layer inside its body configuration to hold up to three instances of Neo.grid.Body.
• Example DOM target:

      <div class="neo-grid-body-wrapper" style="overflow-y: auto;">
      <div class="neo-multi-body-layout" style="display: flex;">
          <div class="neo-body-start">...</div>
          <div class="neo-body-center">...</div>
          <div class="neo-body-end">...</div>
      </div>
  </div>

• removeDom: true Optimization: If a zone (e.g., Left) has no columns assigned to its collection, its corresponding SubGrid VDOM node should receive removeDom: true to keep the live DOM lightweight while preserving the component instance and VDOM structure.

3. Routing Configuration:

• The grid.Container must route the partitioned column definitions to their respective grid.Body instances.

4. Header Splitting:

• Refactor header.Toolbar.mjs to be instantiated up to three times (one per zone), binding to the respective column collections defined in Phase 1.

Note: Fixing the vertical scroll math and row pooling for these bodies will be handled in subsequent tickets.