ES|QL Delta Command: Row-over-row calculations for time-series analysis

## Value Proposition

Users performing time-series analysis need to calculate differences between consecutive events — throughput from cumulative byte counters, time between logs, or rate-of-change detection. Currently this is impossible directly in ES|QL because the language cannot reference a previous row's value.

## Expected Outcome

A new ES|QL command that computes the difference between a numeric field's value in the current row and a preceding row, enabling rate-of-change analysis, spike detection, and flow rate computation directly in the query pipeline.