The Open->HalfOpen promotion used time.Now/time.Since directly, forcing tests
to use real time.Sleep and diverging from the project's clock convention. Add
an unexported withClock option (default clock.System) and replace the real
sleeps in tests with mock-clock Advance, making the transitions deterministic
and the package faster.
Implements circuit breaker as a RoundTripper middleware:
- Closed → Open after consecutive failure threshold
- Open → HalfOpen after configurable duration
- HalfOpen → Closed on success, back to Open on failure
- Per-host tracking via sync.Map for independent endpoint isolation
