tdd-methodology

# Test-Driven Development Methodology

Tests verify specifications are met. Write tests BEFORE implementation. Tests reference spec paragraph IDs for traceability.

## The TDD Cycle

```
┌──────────────────┐
│  1. RED          │  Write a failing test for spec requirement
├──────────────────┤
│  2. GREEN        │  Write minimal code to pass
├──────────────────┤
│  3. REFACTOR     │  Improve code, keep tests green
└──────────────────┘
```

## Four Test Types

### 1. Unit Tests

**Purpose**: Test individual functions/methods in isolation  
**Location**: `tests/unit/`  
**Characteristics**:
- Fast (ms per test)
- No I/O, no network, no filesystem
- Mock external dependencies
- One assertion focus per test

**Pattern**:
```
test_<function>_<scenario>_<expected>
```

**Example** (Rust):
```rust
#[test]
// @trace SPEC-03.07
fn test_validate_length_exceeds_max_returns_error() {
    let input = "x".repeat(1025);
    assert!(validate_length(&input).is_err());
}
```

**Example** (TypeScript):
```typescript
describe('validateLength', () => {
    // @trace SPEC-03.07
    it('returns error when input exceeds max length', () => {
        const input = 'x'.repeat(1025);
        expect(() => validateLength(input)).toThrow();
    });
});
```

**Example** (Python):
```python
def test_validate_length_exceeds_max_returns_error():
    """@trace SPEC-03.07"""
    input_str = 'x' * 1025
    with pytest.raises(ValidationError):
        validate_length(input_str)
```

### 2. Integration Tests

**Purpose**: Test component interactions, real dependencies  
**Location**: `tests/integration/`  
**Characteristics**:
- Slower (seconds per test)
- Real database, real filesystem, test containers
- Test boundaries between modules
- Setup/teardown for external state

**Pattern**:
```
test_<component>_<integration>_<scenario>
```

**Example** (Rust):
```rust
#[tokio::test]
// @trace SPEC-05.12
async fn test_auth_service_validates_against_database() {
    let db = TestDb::new().await;
    let auth = Aut