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/weighted.go

@@ -0,0 +1,42 @@
+package balancer
+
+import "math/rand/v2"
+
+type weightedRandom struct{}
+
+// WeightedRandom returns a strategy that selects endpoints randomly,
+// weighted by each endpoint's Weight field. Endpoints with Weight <= 0
+// are treated as having a weight of 1.
+func WeightedRandom() Strategy {
+	return &weightedRandom{}
+}
+
+func (w *weightedRandom) Next(healthy []Endpoint) (Endpoint, error) {
+	if len(healthy) == 0 {
+		return Endpoint{}, ErrNoHealthy
+	}
+
+	totalWeight := 0
+	for _, ep := range healthy {
+		weight := ep.Weight
+		if weight <= 0 {
+			weight = 1
+		}
+		totalWeight += weight
+	}
+
+	r := rand.IntN(totalWeight)
+	for _, ep := range healthy {
+		weight := ep.Weight
+		if weight <= 0 {
+			weight = 1
+		}
+		r -= weight
+		if r < 0 {
+			return ep, nil
+		}
+	}
+
+	// Should never reach here, but return last endpoint as a safeguard.
+	return healthy[len(healthy)-1], nil
+}