When ip link del removes an interface, the kernel sends RTM_DELLINK after the interface is unregistered from the name table.
if_indextoname fails and handle_nl_link silently returned without calling remove_iface_sockets,
causing old ETCP sockets to accumulate indefinitely.
Fix: fall through to remove_iface_sockets(ifindex) even when if_indextoname fails.
Test: added diagnostics (diag_dump_state, per-category debug levels to file) to verify socket/links cleanup.