A 72-year-old woman with a 20-year history of sun exposure presents with a 1.5-cm tan-brown macule with irregular borders on her left temple. The lesion has been present for 5 years and has slowly enlarged. Dermoscopy shows asymmetrical pigmentation with a rhomboidal pattern and follicular involvement. Excision biopsy reveals atypical melanocytes confined to the epidermis and superficial dermis (Clark level II) with a Breslow thickness of 0.8 mm. Mitotic rate is <1/mm². What is the most likely melanoma subtype, and what is the key prognostic implication?