Function calculate_complexity 

pub fn calculate_complexity(spec: &Spec) -> ComplexityGrade

Calculate complexity grade based on criteria count, target files, and word count

Grading rules:

• Grade A: 1-3 criteria, 1-2 files, <200 words
• Grade B: 4-5 criteria, 3 files, 200-400 words
• Grade C: 6-7 criteria, 4 files, 400-600 words
• Grade D: 8+ criteria OR 5+ files OR 600+ words

If any single metric triggers D, overall grade is D.