Stella Lefty brings a rock/pop set to Mercury Lounge, placing a single voice and band sound in one of the city’s long-running testing grounds for new material. The show unfolds as a straightforward live performance, closer to a working document than a spectacle. In a New York venue known for first looks and early runs, this appearance reads as part of the city’s ongoing cycle of artists sharpening songs in public. The restraint in the staging puts the writing and pacing under clear light.