A 3-year-old child from rural Odisha presents with corneal scarring and blindness. On examination, the cornea is opaque and vascularized. The mother reports the child had night blindness 6 months ago, followed by conjunctival xerosis and a foamy patch on the nasal side. Which clinical feature best distinguishes corneal scarring (Stage X4) from corneal ulceration (Stage X3) in vitamin A deficiency?