feat(qbz-cache): extract audio caching to standalone crate

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.

crates/qbz-cache/Cargo.toml

@@ -0,0 +1,17 @@
+[package]
+name = "qbz-cache"
+version = "0.1.0"
+edition = "2021"
+description = "Audio caching system for QBZ - L1 memory + L2 disk caches"
+authors = ["blitzkriegfc"]
+
+[dependencies]
+# Logging
+log = "0.4"
+
+# Serialization (for stats)
+serde = { version = "1", features = ["derive"] }
+
+# File system utilities
+dirs = "5"
+filetime = "0.2"