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/crates/musicfs-cli/Cargo.toml

@@ -17,6 +17,7 @@ musicfs-metadata.path = "../musicfs-metadata"
 
 clap.workspace = true
 tokio.workspace = true
+tokio-util.workspace = true
 tracing.workspace = true
 tracing-subscriber.workspace = true
 tracing-appender.workspace = true