Write Vitest Unit Tests

For code that already exists. If the user is writing tests before the implementation, stop and use the tdd skill instead — this skill is for backfilling.

File conventions

• One test file per source file, beside the source: src/cart.ts → src/cart.test.ts.
• Use *.test.*, never *.spec.*. In this project .spec. is reserved for integration/e2e tests.
• Mirror the source extension: .ts → .test.ts, .tsx → .test.tsx.
• One describe per exported symbol; nest describe for sub-cases when it improves readability.

Modern Vitest standards