5.1 and 7.1 surround now works end to end (host encodes multichannel
via multistream Opus; native clients render >2 channels via
AudioDec::Surround). Move it to Shipped + the at-a-glance table, and
narrow the Planned entry to the genuinely-future object-based spatial
audio work. Corrects the stale 'every path is stereo end to end / no
client renders it yet' claim.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>