Downtown@HiSAM   In this sparely modern cafe on the ground floor of the Hawai'i State Art Museum, the food echoes that of sister restaurant Town (see below) but doesn't copy it. Grilled 'ahi sandwiches, salads chock-full of cheeses, nuts and fruits, Manila clams in saffron broth are just a few items on the oft-rotating menu. At takeout counter Downtown ASAP! pick up panini filled with combos such as roasted vegetables and goat cheese, and a selection of salads. It's the best fresh deal in town.  Hawaii State Art Museum, 250 S. Hotel St. at Richards St. 536-5900. 7 a.m.-2 p.m. Mondays-Fridays, 11 a.m.-2 p.m. Saturdays. AX, DS, DC, MC, VS. $$.