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/src/traits.rs

@@ -0,0 +1,55 @@
+use async_trait::async_trait;
+use musicfs_core::{DirEntry, FileStat, HealthStatus, OriginId, Result};
+use std::path::{Path, PathBuf};
+use tokio::io::AsyncRead;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum OriginType {
+    Local,
+    Nfs,
+    Smb,
+    S3,
+    Sftp,
+}
+
+#[async_trait]
+pub trait Origin: Send + Sync {
+    fn id(&self) -> &OriginId;
+
+    fn origin_type(&self) -> OriginType;
+
+    fn display_name(&self) -> &str;
+
+    async fn readdir(&self, path: &Path) -> Result<Vec<DirEntry>>;
+
+    async fn stat(&self, path: &Path) -> Result<FileStat>;
+
+    async fn read(&self, path: &Path, offset: u64, size: u32) -> Result<Vec<u8>>;
+
+    async fn exists(&self, path: &Path) -> Result<bool>;
+
+    async fn health(&self) -> HealthStatus;
+
+    async fn open_read(&self, path: &Path) -> Result<Box<dyn AsyncRead + Send + Unpin>>;
+
+    async fn watch(&self, path: &Path, callback: WatchCallback) -> Result<WatchHandle>;
+}
+
+pub type WatchCallback = Box<dyn Fn(WatchEvent) + Send + Sync>;
+
+pub struct WatchHandle {
+    _cancel: tokio::sync::oneshot::Sender<()>,
+}
+
+impl WatchHandle {
+    pub fn new(cancel: tokio::sync::oneshot::Sender<()>) -> Self {
+        Self { _cancel: cancel }
+    }
+}
+
+#[derive(Debug, Clone)]
+pub enum WatchEvent {
+    Created(PathBuf),
+    Modified(PathBuf),
+    Deleted(PathBuf),
+}