A.R. Gurney's whimsical but whippet-thin diversion, 'Sylvia,' is about marriage and midlife and how man's best friend can become a woman's worst enemy...It's just a cute and clever conceit. But not quite enough to sustain two hours. Director Daniel Sullivan has a light touch with his a tightknit ensemble, but the play is undercooked and overlong. Still, there are pleasures. Annaleigh Ashford ('Masters of Sex') makes an adorable talking stray mutt...she affirms her stature as a comic ace...Matthew Broderick gives his most assured performance in a long while as Greg, an unhappily employed middle-aged empty-nester who finds Syliva in the park and becomes obsessed.