A 28-year-old woman presents with a 6-month history of episodic fingertip color changes in response to cold, morning stiffness in her hands, and joint pains. She also reports heartburn and difficulty swallowing solid foods. On examination, she has puffy, swollen fingers with mild skin tightening over the digits, proximal muscle weakness, and diminished distal pulses in cold environments. Laboratory studies reveal an ANA titer of 1:640 with a speckled pattern, negative anti-dsDNA, negative anti-centromere, and strongly positive anti–U1 RNP antibodies. Electromyography demonstrates a myopathic pattern. Which of the following is the most likely diagnosis?