fix: handle tiny values in toFixed

[DeoJin]

Summary

• fix toFixed so exponential-notation inputs are shifted correctly before and after rounding
• add regression coverage for tiny exponential-form values in both Jasmine and QUnit suites
• preserve existing rounding behavior for normal decimal inputs

Testing

• node -e "const accounting=require('./accounting.js'); const cases=[[accounting.toFixed('-0.00000000000000000000003',2),'0.00'],[accounting.toFixed('-0.00000000000000000000003',23),'-0.00000000000000000000003'],[accounting.formatNumber(1e-7,2),'0.00'],[accounting.formatNumber(1e-7,8),'0.00000010'],[accounting.toFixed(0.615,2),'0.62'],[accounting.toFixed(574.425,2),'574.43']]; for (const [actual, expected] of cases) { if (actual !== expected) { console.error(JSON.stringify({actual, expected})); process.exit(1);} } console.log('node-assertions: ok');"

Fixes #233