Single home for the non-trivial form-interceptor enhance() shared by the
four complex consumers: it intercepts submit, invokes the SubmitFunction,
and fires the returned callback with a configurable result. setFormResult()
drives the success/failure branch; an embedded beforeEach resets it before
every test so isolation is structural. Consumed via vi.mock('$app/forms',
() => ({ ...formsMock })) through the $mocks alias. Part of #560.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>