Add 2026 tax config: Japan (jp)

[pascalwhoop]

Overview

Japan's fiscal year 2026 starts April 1, 2026. The current 2025 config covers FY2025. This issue tracks the FY2026 update — relevant from April 2026 onwards.

Key FY2026 changes to research

• Income tax brackets and rates (confirm if unchanged)
• Basic deduction (基礎控除) amount
• Employment income deduction (給与所得控除) tables
• Social insurance rates: health insurance, pension (kosei nenkin), employment insurance
• Resident tax (住民税) standard rates

Acceptance criteria

• configs/jp/2026/base.yaml with accurate FY2026 parameters
• Test vectors pass npm run test:configs
• Sources cited from NTA (National Tax Agency) publications

Implementation

Use the /add-new-country skill. Start from the 2025 config. Target completion before April 1, 2026.