perf(grid): hoist secondary click (context menu) and tooltip handling from _GridCell to grid level #1346

Summary

In #983, primary pointer taps and cursor regions were successfully hoisted to grid-level hit-testing, significantly reducing per-cell overhead during scrolling and column-window shifts. However, as explicitly noted in #983 (comment) and #987, secondary-tap (right-click), ContextMenu (from shadcn_flutter), and material.Tooltip handling remained inside each visible _GridCell.

For a standard viewport with 30 visible rows and 20 visible columns (600 visible cells), this mounts:

• 600 material.Listener(onPointerDown: ...) instances
• 600 ContextMenu instances (with eager child builders)
• 600 material.Tooltip instances (each maintaining internal gesture and overlay timers)

This bloats the widget tree with ~4,200 widgets/render objects and retains a dependency on shadcn_flutter inside the hot grid rendering loop (related to Epic #1331).

Proposed Solution

1. Grid-level right click: Capture secondary pointer down events at the grid gesture layer (GridPointerSelection / grid hit test) and compute _cellAtOffset(offset) on demand.
2. On-demand ContextMenu: Only instantiate and display the context menu when a cell is actually right-clicked, instead of wrapping every visible cell in a ContextMenu.
3. Grid-level Tooltip / Cell Popover: Hoist truncated cell tooltip display to the grid level or trigger it via the existing _GridCellPopoverInspector mechanism.
4. Convert _GridCell to a pure, lightweight leaf widget (Container with padding, border, and Text).

Acceptance Criteria

• _GridCell does not instantiate per-cell ContextMenu, Listener(onPointerDown), or Tooltip.
• Right-clicking any cell continues to open the cell context menu at the click position.
• Hovering over truncated cells continues to show tooltip / value inspection.
• Frame build time during horizontal window shifts (grid_perf_bench.dart MODE=scroll) shows measurable reduction.