Fix auto export row selection sync by using correct AG Grid event

[jlancaster7]

Problem

When export_mode='auto' is set, clicking a row in the grid should sync the selected row data to Python via grid_data_out and fire observe callbacks. Instead, grid_data_out stays empty and no callbacks fire. The grid renders fine and selection highlights visually, but nothing reaches Python.

Reported in #68 and #76.

Root Cause

In js/src/widget_builder.js, the auto export listener for row selection was using AG Grid's rowSelected event instead of selectionChanged:

• rowSelected fires once per affected row during a selection change. In single-selection mode, clicking a new row fires it twice (deselect old + select new). The first firing occurs mid-transition when getSelectedNodes() returns inconsistent state. Both firings call model.set('_grid_data_up', ...) in rapid succession, and traitlets silently drops the update because it doesn't detect a meaningful change.

• selectionChanged fires once after all row selection/deselection is complete. At that point, getSelectedNodes() returns the final, consistent selection state. This is the correct event per AG Grid documentation.

Fix

One-line change in js/src/widget_builder.js:180:

-            gridOptions.api.addEventListener('rowSelected', () => {
+            gridOptions.api.addEventListener('selectionChanged', () => {

Testing

Verified in a Jupyter notebook with the following test cases:

• export_mode='auto' + rowSelection: 'single' — clicking a row fires observe callback on grid_data_out immediately
• export_mode='auto' + rowSelection: 'multiple' — Ctrl+click selects multiple rows, all appear in grid_data_out['rows']
• export_mode='auto' + export_to_df=True — selected rows returned as DataFrame
• export_mode='auto' + export_to_df=False — selected rows returned as list of dicts
• export_mode='buttons' — still works (no regression)
• get_selected_rows() — still works (no regression)

Related Issues

• Grid Selections Not Being Captured #68 — "Grid Selections Not Being Captured": this fix addresses the core export_mode='auto' selection sync failure reported there.
• Reliable user selection real time listener #76 — "Reliable user selection real time listener": this fix addresses the root wiring problem identified through that user's reverse engineering. Note: Reliable user selection real time listener #76 also identified other bugs (a typo in grid.py:241 and grouped row handling limitations) that are separate from this fix and remain open.