Alas, "Forever Fernwood" reads funnier than it plays. Norman Lear's syndicated sequel to "Mary Hartman, Mary Hartman" arrived on TV last week, not only without Louise Lasser's Mary but largely devoid of its predecessor's angst-ridden subtlety. The charm of the original flowed from its skill at meshing soap-opera satire with poignantly vulnerable characters. The second time around, the denizens of Fernwood have sold out their humanity for all-out parody. Lear's split-level soap now reflects not recognizable neuroses but lunatic posturings.