## DOCUMENTATION: Testing Strategy and How to Run Tests

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Description
There is no document explaining the project's testing approach, what frameworks are used, or how to run the test suites. This blocks contributors from writing or running tests confidently.

Tasks

• Create docs/TESTING.md at the repo root.
• Explain the three layers: unit (Vitest + RTL), integration (Vitest + Supertest), and E2E (Cypress).
• Document commands: pnpm test, pnpm test:ci, pnpm cypress:open.
• Add a section on writing new tests (file location conventions, mocking patterns).
• Include a section on the Rust/Soroban contract test workflow.
  Acceptance Criteria
• A contributor can find, run, and write tests for any layer using only this document.