fix(network): prevent near-zero bandwidth on rapid event-driven updates

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>

include/modules/network.hpp

@@ -70,6 +70,8 @@ class Network : public ALabel {
 
   unsigned long long bandwidth_down_total_{0};
   unsigned long long bandwidth_up_total_{0};
+  unsigned long long bandwidth_down_prev_{0};
+  unsigned long long bandwidth_up_prev_{0};
   std::chrono::steady_clock::time_point bandwidth_last_sample_time_;
 
   std::string state_;