Delta / interval timestamps (time since last line)

[gbmhunter]

Summary

Add a new timestamp format that shows the elapsed time since the previous received line (delta/interval), rather than or in addition to the absolute timestamp.

Motivation

Absolute timestamps tell you when something happened; delta timestamps tell you how long the device took to respond. This is essential for:

• Measuring command response times
• Spotting timing anomalies in sensor output
• Debugging timeout issues

Proposed behaviour

• New option in the timestamp format dropdown: Delta (ms since last line)
• Optionally a combined mode showing both absolute and delta (e.g. [12:34:56.789 +342ms])
• Delta resets to 0 on the first line after a period of silence (configurable gap threshold, or just always relative to previous line)

Precedent

IO Ninja (delta timestamps)