A busy node opens roughly three file descriptors per established UDP peer
(a connect()-ed socket plus a 2-FD drain self-pipe), so the default 1024
soft RLIMIT_NOFILE is exhausted near 320 peers and further peer admission,
handshakes, and discovery fail with EMFILE. Document the FD budget, the
symptom, and the systemd (LimitNOFILE drop-in) and OpenWrt (procd nofile)
procedures to raise it, plus how to verify the per-peer ratio is bounded.
Link the new guide from the how-to index.
