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/Cargo.toml

@@ -6,7 +6,8 @@ edition.workspace = true
 [dependencies]
 musicfs-core = { path = "../musicfs-core" }
 async-trait.workspace = true
-tokio = { workspace = true, features = ["fs", "sync"] }
+dashmap.workspace = true
+tokio = { workspace = true, features = ["fs", "sync", "time"] }
 tracing.workspace = true
 
 [dev-dependencies]