Fix a Node.js circuit breaker state machine (rolling window, half-open concurrency, timeouts, exact transition events)

# Task: fix the circuit breaker state machine `breaker.js` (Node 20, CommonJS, no dependencies) wraps calls to a flaky service. Make `CircuitBreaker` implement exactly this state machine (keep the constructor options and export): - **closed**: `call(...args)` invokes `fn(...args)` and resolves/rejects with its result. A synchronous throw from `fn` behaves like a rejection. A failure is a rejection for which `isFailure(err)` returns true (others are passed through to the caller but otherwise treated like a success: not counted). Failures are counted in a **rolling window**: only failures whose time (`now()` when the call settled) is within the last `windowMs` (i.e. `t > now - windowMs`) count; successes do **not** reset the count. When the count reaches `failureThreshold`, the breaker goes **open** (`openedAt = now()`). - **open**: every call rejects immediately with an `Error` whose `code` is `'EOPEN'`, without invoking `fn`, until `now() - openedAt >= cooldownMs`; at that point the breaker is **half_open** (the `state` getter must already report `'half_open'` then, even before a call). - **half_open**: at most `halfOpenMax` trial calls may be in flight at once; extra calls reject with `EOPEN` without invoking `fn`. Any trial failure -> **open** again (new `openedAt`), and the in-flight results don't change state further. After `successThreshold` successful trials in a row -> **closed** with failure history cleared. - `timeoutMs` > 0: a call not settled within `timeoutMs` real milliseconds rejects with an `Error` with `code` `'ETIMEOUT'`, and counts as a failure (a later settlement of that call is ignored). The timer must be cleared when the call settles first. - `on('stateChange', cb)` calls `cb(from, to)` exactly once per actual transition (including open -> half_open), in order; `on` returns `this`. Add tests under `test/` if you want (`node --test test/`). No network, no npm.