Jspreadsheet benchmarks

Partial native-API port: Group, Pivot and collapse benchmarks are disabled, not simulated. Panel clicks use native range selection; row marks also track the noncontiguous benchmark selections. Official evaluation mode is enabled: no signup or personal key is required on localhost. Serve this page from http://localhost or http://127.0.0.1, not file://. Keep the original accounts.js beside this HTML. Data files are loaded from data/, with a same-directory fallback. CDN access is required. Integration and million-row performance still require validation with the actual Jspreadsheet build. Full Printable and millions of merged ranges can exhaust browser memory. XLSX uses a separate native worksheet with materialized values and a real header row; Panel is excluded. Automatic sheet splitting is not implemented.