The API's lifespan unconditionally spawned a MACVLAN sniffer task, which
duplicated the standalone 'decnet sniffer --daemon' process that
'decnet deploy' always starts — causing two workers to sniff the same
interface, double events, and wasted CPU.
Mirror the existing DECNET_EMBED_PROFILER pattern: sniffer is OFF by
default, opt in explicitly. Static regression tests guard against
accidental removal of the gate.
