// Package balancer provides client-side load balancing as HTTP middleware.
//
// It distributes requests across multiple backend endpoints using pluggable
// strategies (round-robin, weighted, failover) with optional health checking.
//
// # Usage
//
//	mw, closer := balancer.Transport(
//	    []balancer.Endpoint{
//	        {URL: "http://backend1:8080"},
//	        {URL: "http://backend2:8080"},
//	    },
//	    balancer.WithStrategy(balancer.RoundRobin()),
//	    balancer.WithHealthCheck(5 * time.Second),
//	)
//	defer closer.Close()
//	transport := mw(http.DefaultTransport)
//
// # Strategies
//
//   - RoundRobin — cycles through healthy endpoints
//   - Weighted — distributes based on endpoint Weight field
//   - Failover — prefers primary, falls back to secondaries
//
// # Health checking
//
// When enabled, a background goroutine periodically probes each endpoint.
// The returned Closer must be closed to stop the health checker goroutine.
// In httpx.Client, this is handled by Client.Close().
//
// # Sentinel errors
//
// ErrNoHealthy is returned when no healthy endpoints are available.
package balancer