Wake: a target may list several broadcast addresses

Config keeps broadcast (one) and adds broadcasts (a list); exactly one must
be present. Wake.Addresses() returns Broadcast then Broadcasts; checkWake
errors on both-set, neither-or-empty, and non-host:port entries. Waker sends
to every address in order, logging/past a failure so one dead address does
not stop the others, and returns false only when none could be sent.

Implemented-By: OpenCode session (model recorded in docs/implementer-log.md)

cmd/crossbar/main.go

@@ -70,7 +70,7 @@ func run() error {
 		if h.Wake == nil {
 			continue
 		}
-		targets[name] = wake.Target{MAC: h.Wake.MAC, Broadcast: h.Wake.Broadcast, Wait: h.Wake.Wait.Duration}
+		targets[name] = wake.Target{MAC: h.Wake.MAC, Broadcasts: h.Wake.Addresses(), Wait: h.Wake.Wait.Duration}
 	}
 	waker := wake.New(targets, hosts)
 	for name, h := range cfg.Hosts {