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/roundrobin.go

@@ -0,0 +1,21 @@
+package balancer
+
+import "sync/atomic"
+
+type roundRobin struct {
+	counter atomic.Uint64
+}
+
+// RoundRobin returns a strategy that cycles through healthy endpoints
+// sequentially using an atomic counter.
+func RoundRobin() Strategy {
+	return &roundRobin{}
+}
+
+func (r *roundRobin) Next(healthy []Endpoint) (Endpoint, error) {
+	if len(healthy) == 0 {
+		return Endpoint{}, ErrNoHealthy
+	}
+	idx := r.counter.Add(1) - 1
+	return healthy[idx%uint64(len(healthy))], nil
+}