Function resolve_wal_ceiling 

pub fn resolve_wal_ceiling(
    backend_field: Option<u64>,
    env_value: Option<&str>,
    backend_name: &str,
    kind: BackendKind,
    wal_mode: bool,
    read_only: bool,
) -> Result<ResolvedWalCeiling, ConfigError>

Resolve the ceiling with backend-field precedence over the environment.

env_value is a construction-time snapshot supplied by the host. This function never reads process environment, so forwarding and backend opening can validate the same policy even when the environment later changes.