Implement Phase B: Crash Recovery

Add startup integrity checks, corruption recovery, CAS size limits,
graceful shutdown orchestration, and a task supervisor — turning 5
previously-RED resilience tests GREEN and adding 5 new tests.

- CAS: pre-check size limit in put(), add StoreFull error variant
- CAS: sled corruption recovery in open() (retry then recreate)
- SQLite: open_with_integrity_check() via PRAGMA integrity_check(1)
- tantivy: open_with_recovery() deletes and rebuilds corrupt index
- CLI: CancellationToken-based ordered shutdown sequence
- Core: TaskSupervisor with spawn_supervised/spawn_critical + backoff
- Tests: replace 4 todo!() stubs, add 5 new shutdown/supervisor tests

musicfs/Cargo.toml

@@ -13,6 +13,7 @@ repository = "https://github.com/user/musicfs"
 [workspace.dependencies]
 # Async runtime
 tokio = { version = "1", features = ["full"] }
+tokio-util = { version = "0.7", features = ["rt"] }
 async-trait = "0.1"
 
 # Error handling