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/src/error.rs

@@ -0,0 +1,30 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum Error {
+    #[error("I/O error: {0}")]
+    Io(#[from] std::io::Error),
+
+    #[error("Origin not found: {0}")]
+    OriginNotFound(String),
+
+    #[error("File not found: {0}")]
+    FileNotFound(String),
+
+    #[error("Path resolution failed: {0}")]
+    PathResolution(String),
+
+    #[error("Cache error: {0}")]
+    Cache(String),
+
+    #[error("Database error: {0}")]
+    Database(String),
+
+    #[error("NFS stale file handle")]
+    NfsStaleHandle,
+
+    #[error("Operation not permitted (read-only filesystem)")]
+    ReadOnly,
+}
+
+pub type Result<T> = std::result::Result<T, Error>;