QoS closed loop and embedded recv foundation

[YanChao1999]

Summary

• Add RTCP APP recv feedback (ZCst) reporting loss, jitter, decode lag, and dropped frames to the sender
• Extend QosController to downgrade on recv feedback in addition to sender encode/fps metrics
• Instrument recv decode timing and queue depth; wire feedback through recv, recv-log, and display paths
• Add StreamPublisher::readvertise() for dynamic mDNS updates and Pi Zero W deployment docs/scripts

Test plan

• cargo test -p zerocast_core qos::
• cargo test -p zerocast_transport --lib rtcp
• Two-terminal smoke: recv-log --profile low + stream --discover --profile auto
• Confirm sender logs qos: recv feedback and ladder downgrade under load
• Optional: Pi Zero W or QEMU ARM recv lab per docs/PI-ZERO-W.md

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