Function check_contig_limit 

pub fn check_contig_limit(count: usize) -> Option<String>

Check if adding another contig would exceed the maximum allowed.

Call this with the current count BEFORE adding a new contig. Returns an error message if adding would exceed the limit, None if safe to add.

Example

if check_contig_limit(contigs.len()).is_some() {
    return Err(...);
}
contigs.push(new_contig); // Safe to add