Test Data Builder

Builds realistic domain fixtures, factories, and edge-case datasets with the builder pattern and valid defaults, so each test owns exactly the data it asserts on. Use when someone asks "how should I set up test data", "my fixtures broke half the suite", "make a factory for this model", "why is this test flaky only in CI", or a test needs domain objects, seed data, or boundary inputs. Do NOT use when a test needs to…