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/tests/integration/docker-compose.yml

@@ -0,0 +1,40 @@
+services:
+  toxiproxy:
+    image: ghcr.io/shopify/toxiproxy:2.9.0
+    ports:
+      - "8474:8474"
+      - "20000-20010:20000-20010"
+    healthcheck:
+      test: ["CMD", "/toxiproxy-cli", "list"]
+      interval: 5s
+      timeout: 3s
+      retries: 3
+
+  minio:
+    image: minio/minio:latest
+    command: server /data --console-address ":9001"
+    ports:
+      - "9000:9000"
+      - "9001:9001"
+    environment:
+      MINIO_ROOT_USER: test
+      MINIO_ROOT_PASSWORD: testtest123
+    healthcheck:
+      test: ["CMD", "curl", "-f", "http://localhost:9000/minio/health/live"]
+      interval: 5s
+      timeout: 3s
+      retries: 3
+    volumes:
+      - minio-data:/data
+
+  sftp:
+    image: atmoz/sftp:latest
+    ports:
+      - "2222:22"
+    command: test:test:::music
+    volumes:
+      - sftp-data:/home/test/music
+
+volumes:
+  minio-data:
+  sftp-data: