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/Cargo.toml

@@ -61,6 +61,12 @@ clap = { version = "4", features = ["derive"] }
 
 # Testing
 tempfile = "3"
+fail = "0.5"
+rlimit = "0.10"
+nix = { version = "0.29", features = ["signal", "process"] }
+wiremock = "0.6"
+assert_cmd = "2.0"
+noxious-client = "1.0"
 
 # Platform-specific
 libc = "0.2"