Gateway detection in the editor previously matched
archetype === 'host-gateway' (a fictional archetype that never
existed in decnet/archetypes.py). Switch to
decky_config.forwards_l3 — the real runtime marker the composer
already reads — so deletion guards, drag-pinning, context menu
locking, and NodeCard DMZ-gateway styling all line up with what
actually ships at deploy time.
On DMZ palette drop, create the gateway with archetype=deaddeck,
services=['ssh'], forwards_l3=true, and mark the edge
is_bridge=true, forwards_l3=true. attachEdge now accepts those
flags so callers can seed a real bridge attachment.
