A 42-year-old man is admitted to the burns unit 4 hours after contact with a hot cooking vessel. He has a 15 cm × 10 cm area on his right forearm that is charred black, leathery, and painless. The edges show a zone of erythema with blistering. On examination, there is no blanching when pressure is applied to the charred area, and a needle prick produces no response in the affected zone. The surrounding erythematous area shows brisk blanching and pain on touch. What is the depth of the primary burn injury in the charred central zone?