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/lib.rs

@@ -1,5 +1,14 @@
+mod failover;
+mod health;
 mod local;
+mod registry;
+mod router;
 mod traits;
 
+pub use failover::{FailoverExecutor, RetryConfig};
+pub use health::{HealthCheckHandle, HealthMonitor, HealthSnapshot, OriginHealthState};
 pub use local::LocalOrigin;
-pub use traits::{Origin, OriginType, WatchCallback, WatchEvent, WatchHandle};
+pub use registry::OriginRegistry;
+pub use router::{LatencyStats, Router};
+pub use musicfs_core::OriginType;
+pub use traits::{Origin, WatchCallback, WatchEvent, WatchHandle};