Function effective_streaming_quality 

pub fn effective_streaming_quality() -> StreamingQuality

The ceiling that actually applies to a stream opened now: the per-playback override if the viewer set one, otherwise the device default.

This is the single resolution point. Every URL builder and the PlaybackInfo negotiation must go through it, for the same reason they all had to agree on the old static: a negotiation that authorises a direct play the URL builder then constrains (or vice versa) leaks the cap.

TRACES: UR-074, UR-079 | DR-226