A 28-year-old man presents with a 3-year history of left ear discharge and progressive hearing loss. On examination, there is a foul-smelling purulent discharge with granulation tissue visible in the external auditory canal. Otoscopy reveals a large posterosuperior marginal perforation. Pure-tone audiometry shows mixed hearing loss (air-bone gap 40 dB, bone conduction threshold 35 dB). CT temporal bone shows erosion of the ossicular chain. What is the most likely diagnosis?