feat: column aggregations (mean, min, max, sum)

[mpryor]

Summary

Add support for basic column aggregation functions — mean/avg, min, max, and sum — for numeric columns.

Details

• Compute aggregations on a selected column and display results (e.g., in a status bar, footer, or popup)
• Should respect active filters (aggregate only visible rows)
• Numeric detection via existing sort coercion logic (int/float)
• Could also support count and count distinct

Possible UX

• Keybinding to show aggregation summary for the current column
• Persistent footer row showing live aggregations for selected columns
• Integration with pivot/grouping — show aggregations per group instead of just count

Considerations

• Non-numeric values: skip or show error count
• Performance on large datasets with streaming data
• Whether aggregations update live as new data arrives in tail mode

[mpryor]

Implemented in 35dfbbe. Press a on any column to see count, distinct, sum, avg, min, max. Respects active filters; skips non-numeric values with a count.