At most one unit test framework

Running Jest and Vitest in the same repo doubles CI runtime because every pipeline executes both runners to cover both test sets. Coverage reports fragment across two tools and merge incorrectly, snapshot formats differ enough to produce false diffs when someone runs the wrong runner, and global setup files (mocks, polyfills, fetch shims) must be duplicated in two configs. Test authors pick whichever syntax they remember, and the suite's actual coverage is impossible to measure accurately.

Pick Vitest for new projects (Jest-compatible API, faster, ESM-native) or stay on Jest if you have heavy jest.mock usage. Migrate config, then convert mocks:

# Replace jest.config.js with vitest.config.ts
# Global sed over test files:
#   jest.fn()   -> vi.fn()
#   jest.mock() -> vi.mock()
npm uninstall jest @types/jest ts-jest babel-jest

Update package.json test scripts and remove Jest-specific setup files from src/__tests__/setup/.