T. PalmerSmitten Ice Cream debuts its quince streusel flavor today.There are ample opportunities to check out some food cart-and-truck action in San Francisco today, whether the hankering is for lunch or dinner. Sam's Chowder Mobile of Half Moon Bay, rarely in S.F. (but still working on building a dedicated city van), will be parked at 500 Terry Francois (at Mission Bay Blvd. N.) from 11:30 a.m. to 1 p.m. Go early for a crack at the item that sells out first, the fantastically buttery lobster