A 28-year-old man from rural Uttar Pradesh presents with a 10-day history of sustained fever (39.5°C), headache, and abdominal discomfort. He reports rose spots on the trunk, splenomegaly on examination, and a relative bradycardia (pulse 78/min despite fever). Blood culture obtained on day 3 of illness grows a Gram-negative rod that is oxidase-negative, ferments glucose and mannitol, produces H₂S, and is motile. The organism is negative for lactose fermentation and VP test. What is the most likely diagnosis?