Function validate_queue_set 

pub fn validate_queue_set(
    active: &QueueFile,
    done: Option<&QueueFile>,
    id_prefix: &str,
    id_width: usize,
    max_dependency_depth: u8,
) -> Result<Vec<ValidationWarning>>

Validates a queue set (active + optional done file).

Returns a vector of warnings if validation succeeds. Warnings are non-blocking issues that should be reported to the user (e.g., dependencies on rejected tasks).