Add mortgage prepayment calculator

[rrih]

Summary

• add a Mortgage Prepayment Calculator at /tools/mortgage-prepayment-calculator/
• compare term-shortening and payment-reduction prepayment scenarios with fixed-rate equal-payment amortization
• add focused Bun tests for monthly payment, amortization, comparison output, and input normalization
• mark mortgage-prepayment-calculator as in_progress in the growth opportunities backlog

Selected action

draft-pr: new high-CPC financial calculator from data/growth/accelerator/latest.md and data/growth/opportunities.json. This is not auto-merged because mortgage repayment estimates are financial calculations and need human review.

Expected KPI impact

Mortgage queries are a higher-CPC utility category than generic developer tools. This adds a new usable calculator page, generated OG image, sitemap entry, and related links to investment / take-home-pay calculators.

Data rationale

The 2026-06-14 Revenue Accelerator report ranked Build Mortgage Prepayment Calculator as priority 99 with high CPC and high potential. Current 28-day PV remains far below the 50,000 JPY/month AdSense target, so high-intent calculator coverage is the selected growth lever.

Source checks

• Japan Housing Finance Agency describes partial prepayment as either keeping monthly payments and shortening the term, or keeping the term and lowering monthly payments: https://www.jhf.go.jp/hensai/kuriage/index.html
• Japanese Bankers Association gives the same repayment-type distinction and notes earlier/larger prepayments have stronger interest-reduction effects: https://www.zenginkyo.or.jp/article/life/myhome/4376/

Verification

• bun test
• bun --bun tsc --noEmit
• bun run lint (passes with existing 9 warnings)
• bun run build
• bun run dev-check
• Browser check: /tools/mortgage-prepayment-calculator/ desktop/default and 390px mobile, no horizontal overflow, input updates recalculated results

Human review focus

• fixed-rate equal-payment assumptions
• yen/month rounding behavior
• prepayment fee and accrued-interest caveats
• tax-credit, rate-reset, bonus-payment, and lender-specific limitations

Refs #15