Add CLI implementation and MVP performance review

- Implement functional CLI with clap argument parsing
- Add directory scanning and metadata extraction at startup
- Fix filesystem.rs to store tokio Handle for async/sync bridge
- Fix flake.nix with LD_LIBRARY_PATH for libfuse3
- Add MVP performance review with real-world benchmark results

Benchmarks show:
- Mount time: 8ms (target <500ms)
- Throughput: 2-3 GB/s (target >500 MB/s)
- Identifies critical gap: incomplete file caching (only ~2MB per file)
- Identifies missing CDC chunking per architecture spec

musicfs/crates/musicfs-cli/Cargo.toml

@@ -8,3 +8,16 @@ name = "musicfs"
 path = "src/main.rs"
 
 [dependencies]
+musicfs-core.path = "../musicfs-core"
+musicfs-origins.path = "../musicfs-origins"
+musicfs-cache.path = "../musicfs-cache"
+musicfs-cas.path = "../musicfs-cas"
+musicfs-fuse.path = "../musicfs-fuse"
+musicfs-metadata.path = "../musicfs-metadata"
+
+clap.workspace = true
+tokio.workspace = true
+tracing.workspace = true
+tracing-subscriber.workspace = true
+anyhow.workspace = true
+dirs.workspace = true