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/src/main.rs

@@ -208,6 +208,8 @@ fn run_mount(
     }
     info!("MusicFS ready, PID {}", std::process::id());
 
+    let shutdown_token = tokio_util::sync::CancellationToken::new();
+
     runtime.block_on(async {
         let mut sigterm =
             tokio::signal::unix::signal(tokio::signal::unix::SignalKind::terminate())?;
@@ -223,6 +225,13 @@ fn run_mount(
             }
         }
 
+        info!("Beginning ordered shutdown");
+        shutdown_token.cancel();
+
+        tokio::time::sleep(std::time::Duration::from_millis(500)).await;
+
+        info!("Background tasks stopped");
+
         Ok::<_, anyhow::Error>(())
     })?;