Add musicfs-test-utils crate with RED resilience tests

Phase 1 of resilience testing design doc implementation:
- New musicfs-test-utils crate with FaultyOrigin, FaultyCasStore, fixtures
- Failpoints instrumented in musicfs-cas/store.rs
- 16 resilience tests (13 RED for missing features, 3 GREEN for existing)
- 3 Docker/Toxiproxy network tests (RED until docker-compose up)
- docker-compose.yml for Toxiproxy + MinIO + SFTP test infrastructure

Tests properly fail-first (TDD): check_all() sequential, no health timeout,
missing corruption detection, no passthrough mode, etc.

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

@@ -180,7 +180,7 @@ impl HealthMonitor {
         HealthCheckHandle { stop_tx }
     }
 
-    async fn check_all(&self) {
+    pub async fn check_all(&self) {
         let origins: Vec<_> = self
             .origins
             .iter()