Add refund helper for review flow

[lyc-aon]

Test PR for the lycaon.wtf Codex review intake E2E path. It intentionally keeps the diff tiny so the automation can review and post safely.

[lyc-aon]

AI-assisted Codex review

Generated from the public diff for #1, requested by @lyc-aon. Please verify findings before merging.

• P1 calculator.js:5
applyDiscountCents no longer enforces the previous 0 <= discountPercent <= 100 contract. With this change, inputs like applyDiscountCents(1000, 150) now return -500, and applyDiscountCents(1000, -10) returns 1100, instead of rejecting invalid discounts. That is a production-impact money calculation regression, and the prior invalid-discount test was removed at the same time, so CI will not catch it.

• P1 calculator.js:13
refundAmountCents accepts any finite numbers and returns Math.min(...) directly, which means it can emit negative refund amounts. For example, refundAmountCents(5000, -100) returns -100. A refund helper should not produce a negative amount for numeric input; it should reject or clamp invalid values. In payment/refund code, allowing negative refund amounts is unsafe and can invert the money flow depending on the caller.

---

Want a Codex review on your PR? Submit a public pull request here: https://lycaon.wtf/#codex-review