A 58-year-old man presents with a 2-cm dark brown nodule on his left shoulder that has been present for 8 months. On dermoscopy, there is a uniform dark color with a well-demarcated border and minimal asymmetry. Histopathology reveals nests of melanoma cells confined to the dermis with no involvement of the epidermis. The lesion shows a vertical growth phase. What is the most likely subtype of melanoma?