Function classify_coverage_severity 

pub fn classify_coverage_severity(
    covered: usize,
    total: usize,
    warn_pct: usize,
    error_pct: usize,
) -> Severity

Classify coverage severity using integer arithmetic (no f64).

Matches the Creusot predicate in gh-verify-verif:

• total == 0 => Pass (no changed lines to cover)
• covered * 100 > warn_pct * total => Pass
• covered * 100 > error_pct * total => Warning
• otherwise => Error