Create qbz-cache crate with:
- AudioCache: L1 in-memory LRU cache (~300MB)
- PlaybackCache: L2 disk-based cache (~500MB)
- LRU eviction with spillover from L1 to L2
src-tauri now re-exports from qbz-cache for backwards compatibility.
This enables TUI/headless clients to use caching without Tauri.