Add load balancer with round-robin, failover, and weighted strategies

Implements balancer middleware with URL rewriting per-request:
- RoundRobin, Failover, and WeightedRandom endpoint selection strategies
- Background HealthChecker with configurable probe interval and path
- Thread-safe health state tracking with sync.RWMutex

balancer/balancer.go

@@ -0,0 +1,78 @@
+package balancer
+
+import (
+	"errors"
+	"net/http"
+	"net/url"
+
+	"git.codelab.vc/pkg/httpx/middleware"
+)
+
+// ErrNoHealthy is returned when no healthy endpoints are available.
+var ErrNoHealthy = errors.New("httpx: no healthy endpoints available")
+
+// Endpoint represents a backend server that can handle requests.
+type Endpoint struct {
+	URL    string
+	Weight int
+	Meta   map[string]string
+}
+
+// Strategy selects an endpoint from the list of healthy endpoints.
+type Strategy interface {
+	Next(healthy []Endpoint) (Endpoint, error)
+}
+
+// Transport returns a middleware that load-balances requests across the
+// provided endpoints using the configured strategy.
+//
+// For each request the middleware picks an endpoint via the strategy,
+// replaces the request URL scheme and host with the endpoint's URL,
+// and forwards the request to the underlying RoundTripper.
+//
+// If active health checking is enabled (WithHealthCheck), a background
+// goroutine periodically probes endpoints. Otherwise all endpoints are
+// assumed healthy.
+func Transport(endpoints []Endpoint, opts ...Option) middleware.Middleware {
+	o := &options{
+		strategy: RoundRobin(),
+	}
+	for _, opt := range opts {
+		opt(o)
+	}
+
+	if o.healthChecker != nil {
+		o.healthChecker.Start(endpoints)
+	}
+
+	return func(next http.RoundTripper) http.RoundTripper {
+		return middleware.RoundTripperFunc(func(req *http.Request) (*http.Response, error) {
+			healthy := endpoints
+			if o.healthChecker != nil {
+				healthy = o.healthChecker.Healthy(endpoints)
+			}
+
+			if len(healthy) == 0 {
+				return nil, ErrNoHealthy
+			}
+
+			ep, err := o.strategy.Next(healthy)
+			if err != nil {
+				return nil, err
+			}
+
+			epURL, err := url.Parse(ep.URL)
+			if err != nil {
+				return nil, err
+			}
+
+			// Clone the request URL and replace scheme+host with the endpoint.
+			r := req.Clone(req.Context())
+			r.URL.Scheme = epURL.Scheme
+			r.URL.Host = epURL.Host
+			r.Host = epURL.Host
+
+			return next.RoundTrip(r)
+		})
+	}
+}