Circuit Breaker

The circuit breaker pattern prevents cascading failures by "tripping" after
consecutive errors and blocking calls while the downstream service recovers.

Implement createCircuitBreaker(threshold, cooldown):

• threshold — consecutive failures to open the circuit.
• cooldown — number of blocked calls before a test call is allowed.

States:

• CLOSED — pass calls through; count consecutive failures.
• OPEN — block calls; count blocked calls. After cooldown blocked calls → HALF_OPEN.
• HALF_OPEN — let one test call through:

- Success → CLOSED (reset failure counter).
- Failure → OPEN (reset cooldown counter).

cb.call(fn) — fn is () => result (throws on failure).
Returns the fn result, or throws 'CIRCUIT_OPEN' when blocked.