Function open_multi_graph_state 

pub async fn open_multi_graph_state(
    graphs: Vec<GraphStartupConfig>,
    tokens: Vec<(String, String)>,
    server_policy_source: Option<&PolicySource>,
    config_path: PathBuf,
    require_all_graphs: bool,
) -> Result<AppState>

Parallel open of every graph in the startup config, with bounded concurrency (buffer_unordered(4)). Graph-specific open failures quarantine that graph; startup succeeds as long as at least one graph opens.

The bound 4 is a rule-of-thumb for I/O-bound work. At N ≤ 10 this trades startup latency for a small amount of concurrent S3 / Lance open pressure.