A 52-year-old woman presents to the emergency department with sudden-onset severe headache, neck stiffness, and photophobia. Non-contrast CT brain is performed urgently. The structure marked **A** (suprasellar cistern) shows hyperdensity replacing the normally dark CSF signal, with similar hyperdensity noted in the interhemispheric fissure and ambient cistern. Which of the following is the most likely diagnosis and the primary vascular source of bleeding in this patient?