Boston’s South Asian food scene has evolved mightily since the bad old days of nothing-but-Punjabi-and-vindaloos of 20 years ago, but it took a while for Sri Lankan cuisine to arrive. This Porter Square spot is a fine introduction to its occasionally fiery wonders, like the edible scoops that are “string hoppers” (little noodle nests) for scooping up savory meat and vegetable stews, the Sri Lankan variant of Southern-style dosas and the novel flatbread that is ground rice-and-grated coconut roti.