Function parse_max_parallel 

pub fn parse_max_parallel(
    value: Option<Value>,
    cap: usize,
) -> Result<usize, CoreError>

Resolve a max_parallel selector into an integer concurrency limit.

Accepts either a positive integer or a string alias:

• serial, sync, synchronous -> 1
• parallel, fan-out, swarm, distributed -> cap
• auto or empty -> cap