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)
Alexander
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