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-origins/Cargo.toml

@@ -0,0 +1,13 @@
+[package]
+name = "musicfs-origins"
+version.workspace = true
+edition.workspace = true
+
+[dependencies]
+musicfs-core = { path = "../musicfs-core" }
+async-trait.workspace = true
+tokio = { workspace = true, features = ["fs", "sync"] }
+tracing.workspace = true
+
+[dev-dependencies]
+tempfile.workspace = true