Add Per-App Audio Streaming for Virtual Display in Scrcpy

Is your feature request related to a problem? Please describe.
Scrcpy supports virtual display (--new-display) for app screen mirroring 🎥, but there’s no built-in way to stream only a specific app’s audio 🔊.

On Android 15, native per-app screen casting with audio is supported without ADB. Adding this to scrcpy would make it even more powerful! 🚀

Describe the solution you'd like
A new option like:
--vd-audio="{app package name || app name}"
This would allow users to stream only a specific app’s audio alongside its screen, keeping it separate from system sounds. 🎧

Additional context
This would be huge for streaming, testing, and content creation! 🎬🎮 Hope to see this added soon! 🙌

Thank you for creating such an amazing tool—scrcpy has been a game-changer! 🙌

Start by tracing the existing --new-display option and the Android 15 per-app screen-casting and audio behavior described in the issue. Determine how a package name or app name would select the audio source and how it would accompany the virtual display. Done means a documented option can stream only the selected app's audio alongside its virtual-display video without system sounds.