Perched on Houston Street, the unofficial border between Manhattan’s East Village and Lower East Side, the Mercury Lounge is a venerable New York City music room with a top-notch sound system. It’s also quite small. Two hundred and fifty people max would be a generous capacity estimate, and the club’s bookings, fittingly, tend toward the alternative and the fringe. So it’s not the first place you’d expect to see a celebrated bluegrass acoustic guitar virtuosa and two-time Grammy winner. But on this extra-warm evening in late June, Molly Tuttle is specializing in the unexpected.