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-test-utils/src/lib.rs

@@ -0,0 +1,9 @@
+pub mod assertions;
+pub mod faulty_cas;
+pub mod faulty_origin;
+pub mod fixtures;
+
+pub use assertions::*;
+pub use faulty_cas::FaultyCasStore;
+pub use faulty_origin::{FailMode, FaultyOrigin};
+pub use fixtures::*;