Add Cursor rule: enforce TDD workflow

[r2DoesInc]

Summary

Add a Cursor rule (.cursor/rules/mdc-tdd-workflow.mdc) that enforces test-driven development whenever possible.

Rule details

The rule should mandate a Red-Green-Refactor cycle for all non-trivial changes:

1. Red — write a small, focused test that fails.
2. Green — write the minimum code to make it pass.
3. Refactor — clean up without changing behavior; tests stay green.

When to apply TDD

• New functions, utilities, helpers
• Bug fixes (reproduce with a test first)
• API route handlers and data-processing logic
• Parsers, validators, transformation code

When TDD is impractical

• Pure layout/styling changes with no logic
• One-line config tweaks
• Temporary spikes (flagged as such)

Test quality guidelines

• One assertion per behavior; many small tests over few large ones
• Name tests as sentences: it("returns 0 when the cart is empty")
• Test inputs/outputs, not implementation details
• Mock external services; never hit real APIs in unit tests
• Keep test files next to source (src/lib/foo.ts → src/lib/__tests__/foo.test.ts)

Acceptance criteria

• .cursor/rules/mdc-tdd-workflow.mdc exists with alwaysApply: true
• Rule documents the Red-Green-Refactor cycle with examples
• Rule lists when TDD applies and when it can be skipped
• Rule includes test quality guidelines