Learns multicast-router ports from observed IGMP membership queries
(struct bridge_mdb_router, keyed by VLAN, aged independently of listener
state) and extends the forwarding path to deliver learned groups to
listener ports and to router ports on the same VLAN, so pruning does not
blackhole traffic a router needs to relay. Adds the mdb_routers counter.

Router forwarding is additive: it can only add destinations to a frame that
was already going to be pruned, never remove one. With no router ports
learned, delivery is unchanged from D4.

Depends on D58744

mdb_router_forward ATF test: in a 4-jail topology, a router port (queries
seen, no join) receives a learned group while a genuinely idle port stays
pruned. Two bugs were found and fixed during VM verification, both in the
test scaffolding rather than the kernel change: a missing <sys/types.h>
include and a missing multicast route for the router jail. Full
regression 21/23 after fixes (+1 vs. D5). Built and booted on both bases;
ATF passes.