feat: audio system (chiptune SFX + dynamic music)

## Summary
Implement audio layer with tone-arc progression.

## Scope
- SFX: shoot, enemy death, hit, boss phase change, scrap pickup
- Music: early-sector chiptune track, mid-sector lo-fi synth, late-sector horror ambient
- Dynamic crossfade between tracks based on sector depth

## Notes
- See DESIGN.md tone arc section
- Depends on #1

Start with the tone arc section in DESIGN.md and review issue #1, which this work depends on. Identify the project's existing audio entry points before implementing the listed SFX, three sector-based music styles, and dynamic crossfade; done means all scoped sounds and music transitions work across sector depth.

Written by the indexing model from the issue text.