fix: disable browser DSP on screen share audio and harden ICE stats resolution
Screen share audio was muffled/gated because getDisplayMedia used plain
`audio: true`, letting the browser apply voice-optimized DSP (NS, AEC, AGC)
to desktop audio. Now passes explicit constraints disabling all processing
and requesting stereo capture.
Also improves WebRTC stats: three-tier ICE candidate-pair discovery
(transport → active-bytes heuristic → legacy fallback), height-based
simulcast layer inference, and documents mDNS obfuscation limitation.
Jannis Braun
2342396fce