Implement Week 6 Origin Federation with Oracle fixes

New files:
- musicfs-core/src/config.rs: Config, OriginConfig, HealthConfig
- musicfs-origins/src/registry.rs: OriginRegistry with watch cleanup
- musicfs-origins/src/router.rs: Priority router with (priority, latency) ordering
- musicfs-origins/src/health.rs: HealthMonitor with per-origin-type thresholds
- musicfs-origins/src/failover.rs: FailoverExecutor with NFR-7.3 backoff

Oracle fixes applied:
- Per-OriginType threshold: Local=1, Remote=3 (check_one uses threshold_for)
- AllOriginsUnhealthy event: Added to events.rs, emitted in select_with_fallback
- Unified OriginType: Removed duplicate from traits.rs, use musicfs_core::OriginType
- Watch handle cleanup: Tracked and dropped on unregister()
- Retry delays: 100ms, 500ms, 2000ms (NFR-7.3 compliant)

Tests: 91 pass (+20 new)

musicfs/crates/musicfs-origins/src/traits.rs

@@ -1,17 +1,8 @@
 use async_trait::async_trait;
-use musicfs_core::{DirEntry, FileStat, HealthStatus, OriginId, Result};
+use musicfs_core::{DirEntry, FileStat, HealthStatus, OriginId, OriginType, 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;