compileViaGrant only handled *Prefix destinations, skipping
*AutoGroup entirely. This meant via grants with
dst=[autogroup:internet] produced no filter rules even when the
node was an exit node with approved exit routes.
Switch the destination loop from a type assertion to a type switch
that handles both *Prefix (subnet routes) and *AutoGroup (exit
routes via autogroup:internet). Also check ExitRoutes() in
addition to SubnetRoutes() so the function doesn't bail early
when a node only has exit routes.
Updates #2180
66ac9a26ff