MatchFromFilterRule only read DstPorts[].IP into the destination
IPSet. Cap-grant-only filter rules (e.g. tailscale.com/cap/relay)
carry their destinations in CapGrant[].Dsts, so the derived matchers
had empty dest sets and BuildPeerMap / ReduceNodes never exposed the
cap target to its source nodes. Without a companion IP-level grant
the relay node stayed invisible, so clients never tried to use it
and connections sat on DERP.
Union CapGrant[].Dsts into the destination IPSet alongside DstPorts.
Restores peer-visibility for any cap-grant-only relationship; the
peer-relay flow is the most visible instance.
Fixes#3256
c3df84e354