When netlink events (link/addr/route changes) fire between timer
intervals, dp.emit() triggers update() which consumes the byte delta
and resets bandwidth_down_total_. A subsequent timer update sees
near-zero delta, displaying very small bandwidth.
Cache the last computed bandwidth values and skip recalculation
when update() is called within half the interval. Event-driven
updates reuse the cached values instead.
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>
