chore: import upstream snapshot with attribution

rules/common/testing.md

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+# Testing Requirements
+
+## Minimum Test Coverage: 80%
+
+Test Types (ALL required):
+1. **Unit Tests** - Individual functions, utilities, components
+2. **Integration Tests** - API endpoints, database operations
+3. **E2E Tests** - Critical user flows (framework chosen per language)
+
+## Test-Driven Development
+
+MANDATORY workflow:
+1. Write test first (RED)
+2. Run test - it should FAIL
+3. Write minimal implementation (GREEN)
+4. Run test - it should PASS
+5. Refactor (IMPROVE)
+6. Verify coverage (80%+)
+
+## Troubleshooting Test Failures
+
+1. Use **tdd-guide** agent
+2. Check test isolation
+3. Verify mocks are correct
+4. Fix implementation, not tests (unless tests are wrong)
+
+## Agent Support
+
+- **tdd-guide** - Use PROACTIVELY for new features, enforces write-tests-first
+
+## Test Structure (AAA Pattern)
+
+Prefer Arrange-Act-Assert structure for tests:
+
+```typescript
+test('calculates similarity correctly', () => {
+  // Arrange
+  const vector1 = [1, 0, 0]
+  const vector2 = [0, 1, 0]
+
+  // Act
+  const similarity = calculateCosineSimilarity(vector1, vector2)
+
+  // Assert
+  expect(similarity).toBe(0)
+})
+```
+
+### Test Naming
+
+Use descriptive names that explain the behavior under test:
+
+```typescript
+test('returns empty array when no markets match query', () => {})
+test('throws error when API key is missing', () => {})
+test('falls back to substring search when Redis is unavailable', () => {})
+```