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/flake.nix

@@ -18,7 +18,7 @@
         };
       in
       {
-        devShells.default = pkgs.mkShell {
+        devShells.default = pkgs.mkShell rec {
           buildInputs = with pkgs; [
             rustToolchain
             pkg-config
@@ -26,20 +26,18 @@
             sqlite
             openssl
             
-            # Linker toolchain
             clang
             lld
             
-            # Dev tools
             cargo-watch
             cargo-nextest
             cargo-criterion
             
-            # gRPC tooling (Week 10+)
             protobuf
             grpcurl
           ];
           
+          LD_LIBRARY_PATH = pkgs.lib.makeLibraryPath buildInputs;
           RUST_BACKTRACE = "1";
           RUST_LOG = "debug";
         };