monitor: Add latency standard deviation

The latency range and moving average alone do not tell a link that is
consistently slow apart from one that is unstable. A high average with a
low deviation points at the scheduling or the interval configuration,
while a low average with a high deviation points at interference,
retransmissions or controller buffer stalls.

Track the sample count along with the sum and the sum of the squares in
struct packet_latency, and report the resulting standard deviation on
the latency lines of both the live decoding and the analyze mode.

The samples are squared in usec and only scaled down to msec^2
afterwards, so that the sub-msec resolution is not lost while the
running sum stays clear of an overflow.

The model derived the accumulator layout and the scaling, which the
author reviewed and verified against an independently computed
deviation.

Assisted-by: opencode:claude-opus-5