A 21-day-old male infant born to a 32-year-old mother presents with poor weight gain (currently 200 g below birth weight), frequent crying, and mother reports 'not enough milk.' On history, the mother initiated breastfeeding on day 1, feeds every 4–5 hours (6 times per day), and has not expressed milk or pumped. Breast examination reveals soft, non-engorged breasts with normal nipples. Infant latch is observed to be deep and correct, with audible swallowing during feeds. The infant passes 4–5 wet diapers and 2–3 stools per day. What is the most likely diagnosis?