Function decode_bgm_to_wav_bytes 

pub fn decode_bgm_to_wav_bytes(
    input: impl AsRef<Path>,
    entry_idx: Option<usize>,
) -> Result<BgmDecoded>

Decode various Siglus audio containers into WAV (PCM16).

• For .ovk, entry_idx selects which entry to decode.
• For other formats, entry_idx is ignored.