feat(audio): add bounded libopus packet core

[rldyourmnd]

Summary

• replace the audio scaffold with a bounded rds-audio packet/codec core
• pin the official libopus reference through opus 0.4.0/bundled opusic-sys
• enforce RFC packet limits, standard frame sizing, wire conversion and PLC
• add sequence reorder/jitter handling, explicit start sequence and fail-closed tests
• document completion workstreams and keep device adapters/service admission explicitly open

Validation

• cargo fmt --check
• cargo test -p rds-audio -p rds-core
• cargo clippy --workspace --all-targets -- -D warnings
• cargo deny check