monitor: Add ISO packet loss counters

Isochronous streams are the one place where HCI hands the host enough
information to tell whether data actually arrived: the ISO data packet
header carries an SDU sequence number and a packet status flag.

Track both per connection and report them during the live decoding and
in the analyze mode. Gaps in the sequence number are counted as lost
SDUs, a packet status flag of 2 is counted as dropped and a flag of 1 as
possibly invalid. Duplicates and reordered sequence numbers are not
counted as loss.

During the live decoding the counters are only printed on the packet
where a discontinuity is detected, so a clean stream produces no extra
output.

The model wrote the sequence number accounting and the ISO header
walking, which the author reviewed and verified against a trace with
known gaps.

Assisted-by: opencode:claude-opus-5