By ALAN FEUER
The neighborhood restaurant plays a limited, but critical, role in New York’s culinary life: It is the place one goes not to be surprised, but satisfied. It’s always a little surprising, then, to find one that’s more than satisfactory. Saggio, a homey, full-flavored trattoria at 829 West 181st Street, is one of the newer restaurants in the micro-neighborhood that sits on the downslope between Fort Washington Avenue and the Henry Hudson Parkway in Washington Heights. On a late spring evening, you will find its front window invitingly thrown open to homebound commuters, congregating dog owners and perhaps a group of teachers from a local elementary school relaxing after a hard day with some pasta and a glass of wine.