Function joint_parallelism_ceiling_for 

pub fn joint_parallelism_ceiling_for(max_concurrency: usize) -> usize

Per-process fan-out width still allowed once max_concurrency processes are counted against max_total_llm_workers.

Pure and total: max_concurrency of 0 is read as 1, and the result never drops below 1 — a joint cap that forbade all work would be a deadlock, not a safety bound.