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/.cargo/config.toml

@@ -0,0 +1,10 @@
+[build]
+rustflags = ["-C", "link-arg=-fuse-ld=lld"]
+
+[target.x86_64-unknown-linux-gnu]
+linker = "clang"
+
+[alias]
+t = "test"
+c = "check"
+b = "build"