A 48-year-old man with a severe subarachnoid haemorrhage (WFNS grade 4, Fisher grade 4) develops new-onset left arm weakness and decreased consciousness on day 6. CT head shows no rebleeding and no hydrocephalus. CT angiography shows narrowing of the right middle cerebral artery. Transcranial Doppler shows a mean flow velocity of 180 cm/s in the right MCA (normal <120 cm/s). What is the most likely diagnosis and initial management?