How I stream Mux video from Sanity to Next.js with signed HLS playback

Sanity Mux video playback in Next.js is where most tutorials stop short. The upload side — pushing a file to Mux via Sanity Studio — is well documented. What I rarely see covered is the read path: pulling a stored Mux asset reference out of Sanity, generating a signed playback URL, initialising hls.