feat: add deterministic original-audio stream contract

[Priveetee]

Summary

• extend /streams audio contract with explicit isOriginal, originalAudioTrackId, and preferredDefaultAudioTrackId
• resolve default audio track deterministically (original -> fallback language -> first valid track id)
• align DASH/native manifest audio ordering with the resolved preferred track when a track id is available

Validation

• JAVA_HOME=/home/ark/.jdks/jdk-17.0.18+8 ./gradlew test