Function shard_count_from 

pub fn shard_count_from(
    env_shards: Option<&str>,
    available_parallelism: usize,
) -> usize

The shard-count law for the uring engine: LOGOS_HTTP_SHARDS when it parses to a positive count (clamped to 1024), else the box’s available parallelism, else 1. Pure so it is testable without touching the process environment.