Add subtotal rows to stats command output

[vertix]

Summary

• Adds hierarchical subtotal rows to the stats command when grouping by multiple fields
• Sorts output alphabetically (instead of by count) so subtotals appear at group boundaries
• Prints subtotals at each grouping level, including a grand total

Test plan

• Run stats command with multiple --fields and verify subtotal rows appear at each group boundary
• Requires eval dataset — not verified locally