Replace the REQ-video-ring-buffer bullet to bind the new
getDisplayMedia + offscreen-document acquisition path and drop the
'active-tab' wording (the new API is screen/window-scoped, not
tab-scoped, so there is nothing to re-attach on tab switch). Encoding,
buffer window, and SPEC §10 acceptance citations are unchanged. Adds
CON-display-capture-binding alongside the existing constraint bindings.
