resilience-patterns

# Resilience Patterns Production-grade patterns for surviving failures without cascading. ## Circuit Breaker ```typescript // States: CLOSED (normal) → OPEN (blocking) → HALF_OPEN (testing) type CircuitState = 'CLOSED' | 'OPEN' | 'HALF_OPEN' class CircuitBreaker { private state: CircuitState = 'CLOSED' private failureCount = 0 private lastFailureTime = 0 private successCount = 0 constructor( private readonly failureThreshold = 5, private readonly recoveryTimeout = 30_000, // ms private readonly halfOpenMaxCalls = 3 ) {} async call<T>(fn: () => Promise<T>): Promise<T> { if (this.state === 'OPEN') { if (Date.now() - this.lastFailureTime > this.recoveryTimeout) { this.state = 'HALF_OPEN' this.successCount = 0 } else { throw new Error('Circuit breaker is OPEN — request rejected') } } try { const result = await fn() this.onSuccess() return result } catch (err) { this.onFailure() throw err } } private onSuccess(): void { if (this.state === 'HALF_OPEN') { this.successCount++ if (this.successCount >= this.halfOpenMaxCalls) { this.state = 'CLOSED' this.failureCount = 0 } } else { this.failureCount = 0 } } private onFailure(): void { this.failureCount++ this.lastFailureTime = Date.now() if (this.failureCount >= this.failureThreshold) { this.state = 'OPEN' } } getState(): Cir...