Implement Week 1 foundation: workspace, core types, FUSE skeleton, LocalOrigin

- musicfs-core: OriginId, FileId, VirtualPath, ContentHash, AudioMeta,
  FileMeta, EventBus with FileAccessed event (5 tests)
- musicfs-fuse: FUSE skeleton with EROFS handlers for write ops
- musicfs-origins: Origin trait with watch(), LocalOrigin impl (6 tests)
- flake.nix: Nix dev shell with rust toolchain, clang, lld, fuse3

All 11 tests pass. Build produces no warnings.

musicfs/crates/musicfs-cli/src/lib.rs

@@ -0,0 +1 @@
+#![allow(dead_code)]

musicfs/crates/musicfs-cli/src/main.rs

@@ -0,0 +1,3 @@
+fn main() {
+    println!("MusicFS CLI - placeholder");
+}