// Package circuitbreaker provides a per-host circuit breaker as HTTP middleware.
//
// The circuit breaker monitors request failures and temporarily blocks requests
// to unhealthy hosts, allowing them time to recover before retrying.
//
// # State machine
//
//   - Closed — normal operation, requests pass through
//   - Open — too many failures, requests are rejected with ErrCircuitOpen
//   - HalfOpen — after a cooldown period, one probe request is allowed through
//
// # Usage
//
//	mw := circuitbreaker.Transport(
//	    circuitbreaker.WithThreshold(5),
//	    circuitbreaker.WithTimeout(30 * time.Second),
//	)
//	transport := mw(http.DefaultTransport)
//
// The circuit breaker is per-host: each unique request host gets its own
// independent breaker state machine stored in a sync.Map.
//
// # Sentinel errors
//
// ErrCircuitOpen is returned when a request is rejected because the circuit
// is in the Open state.
package circuitbreaker