Add netem.sh for simulating adverse network conditions (delay, loss,
jitter, duplication, reordering, corruption) on Docker test containers
using tc/netem. Includes three presets (lossy, congested, terrible) and
apply/remove/status actions.
Fix iperf-test.sh bug where all tests connected the client to its own
FIPS npub (loopback) instead of the remote server's npub, meaning
previous iperf results measured loopback performance rather than
cross-node throughput.
Document netem.sh in README.md.
