- Add input validation (TypeError) to cycleMonth and cycleYear
- Convert tests to node:test runner with describe/it structure
- Add ESLint + Prettier with lint, format, and format:check scripts
- Update CI workflow with lint job, frozen-lockfile, and pack-check
- Add noImplicitReturns, noFallthroughCasesInSwitch, skipLibCheck to tsconfig
- Add invalid Date tests for both ESM and CJS suites