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/options.go

@@ -0,0 +1,25 @@
+package balancer
+
+// options holds configuration for the load balancer transport.
+type options struct {
+	strategy      Strategy       // default RoundRobin
+	healthChecker *HealthChecker // optional
+}
+
+// Option configures the load balancer transport.
+type Option func(*options)
+
+// WithStrategy sets the endpoint selection strategy.
+// If not specified, RoundRobin is used.
+func WithStrategy(s Strategy) Option {
+	return func(o *options) {
+		o.strategy = s
+	}
+}
+
+// WithHealthCheck enables active health checking of endpoints.
+func WithHealthCheck(opts ...HealthOption) Option {
+	return func(o *options) {
+		o.healthChecker = newHealthChecker(opts...)
+	}
+}