Function render_dual_report_html 

pub fn render_dual_report_html(
    source1_name: &str,
    source1: &ReportData,
    source2_name: &str,
    source2: &ReportData,
    calc: &PricingCalculator,
) -> String

Generate a dual-source HTML dashboard with top-level source switcher. Each source gets its own sub-nav with 3 tabs.