refactor(audio): enhance BGM streaming playback

[dogo]

Fix BGM audio playback freeze and noise issues

Issue reported here: dogo/gods-defense#2

Problem

BGM (IMA-ADPCM) audio caused freezes/crashes and had noise/static at start, end, and during loops.

Root Causes

1. Unpacked header struct - BGM_FORMAT_HEADER had padding, misaligning the 53-byte file header
2. Header included in audio data - RAM mode read from file position 0 instead of after header
3. Mono format issues - PSP audio system and double-buffering work better with stereo output

Changes

• bgm.h: Added #pragma pack for exact 53-byte struct
• bgm.c: Fixed file seek to skip header; output stereo (L=R) instead of mono; simplified decoder
• audio.c: Added mono/stereo aware buffer allocation and pointer arithmetic