AI coding standards - OOP, testing

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## OOP Best Practices

### Composition & DI

- Favor composition over inheritance (prefer constructor injection over deep inheritance trees)
- Inject dependencies via constructors or factories
- Avoid service locators or hidden globals

### SOLID Principles

- Always follow SOLID principles

### Domain Modeling

- Use value objects for invariants (e.g., `Email`, `Money`)
- Encapsulate invariants and validation in the model
- Prefer rich domain methods over anemic data structures
- Combat primitive obsession: wrap primitives with validation

### Error Handling

- Create typed error hierarchies with discriminants
- Never throw strings; use `Error` subclasses
- Bubble domain errors, map at boundaries (HTTP/UI)

### Patterns

- Tell, Don't Ask: call behavior methods directly, don't inspect then mutate
- Law of Demeter: interact only with immediate collaborators, avoid long call chains
- Make impossible states impossible: encode valid states in types (discriminated unions)
- Replace conditionals with polymorphism: use Strategy pattern over if/else chains
- Fail fast: use guard clauses for preconditions
- Observers & Events: decouple publishers from subscribers

### Testing

- Design for testability: use constructor DI so test doubles can be injected without mocking frameworks
- Test public behavior, not private implementation
- Mock at architectural boundaries; avoid over-mocking internals

### Heuristics

- Use inheritance only for true is-a relationships
- Permit minimal getters when necessary; encapsulate decisions in methods
- If strict adherence impairs clarity, annotate exceptions with rationale