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-core/src/error.rs

@@ -28,6 +28,15 @@ pub enum Error {
 
     #[error("Operation not permitted (read-only filesystem)")]
     ReadOnly,
+
+    #[error("No origin available to serve request")]
+    NoOriginAvailable,
+
+    #[error("Maximum retries exceeded")]
+    MaxRetriesExceeded,
+
+    #[error("Origin error: {0}")]
+    Origin(String),
 }
 
 pub type Result<T> = std::result::Result<T, Error>;