Lets a client ask "is peer X currently online in network Y?" without
joining that network first — a stateless O(1) lookup against the
anchor's existing client registry. Scoped to (net, id) rather than a
global lookup by id alone, so it can't be used as a cross-network
"is this pubkey online anywhere" oracle: the caller must already know
a network the peer belongs to, matching the knowledge already required
to join it and observe presence the slow way via peer-join/peer-leave.
Motivated by flit's known-devices list, where each pairing is its own
isolated anchor network and the app holds no persistent connection
while idle — today the only way to know if a device is reachable is
to attempt a full connect.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
586d39e3b8