Turn on CMCD: make your CDN logs explain rebuffering

TL;DR CMCD (Common Media Client Data, CTA-5004) lets your player attach its state (buffer length, bitrate, "I just rebuffered") to every segment request, so your CDN's own logs can explain QoE. We'll enable it in hls.js and Shaka, choose header vs query mode, and write a tiny Node parser that turns