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-core/Cargo.toml

@@ -0,0 +1,11 @@
+[package]
+name = "musicfs-core"
+version.workspace = true
+edition.workspace = true
+
+[dependencies]
+thiserror.workspace = true
+serde.workspace = true
+tokio = { workspace = true, features = ["sync"] }
+xxhash-rust.workspace = true
+hex = "0.4"