Add dbx library: PostgreSQL cluster with master/replica routing, retry, health checking

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

balancer.go

@@ -0,0 +1,35 @@
+package dbx
+
+import "sync/atomic"
+
+// Balancer selects the next node from a list.
+// It must return nil if no suitable node is available.
+type Balancer interface {
+	Next(nodes []*Node) *Node
+}
+
+// RoundRobinBalancer distributes load evenly across healthy nodes.
+type RoundRobinBalancer struct {
+	counter atomic.Uint64
+}
+
+// NewRoundRobinBalancer creates a new round-robin balancer.
+func NewRoundRobinBalancer() *RoundRobinBalancer {
+	return &RoundRobinBalancer{}
+}
+
+// Next returns the next healthy node, or nil if none are healthy.
+func (b *RoundRobinBalancer) Next(nodes []*Node) *Node {
+	n := len(nodes)
+	if n == 0 {
+		return nil
+	}
+	idx := b.counter.Add(1)
+	for i := 0; i < n; i++ {
+		node := nodes[(int(idx)+i)%n]
+		if node.IsHealthy() {
+			return node
+		}
+	}
+	return nil
+}