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

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

tx_test.go

@@ -0,0 +1,25 @@
+package dbx
+
+import (
+	"context"
+	"testing"
+)
+
+func TestInjectExtractQuerier(t *testing.T) {
+	ctx := context.Background()
+	fallback := &Node{name: "fallback"}
+
+	// No querier in context → returns fallback
+	got := ExtractQuerier(ctx, fallback)
+	if got != fallback {
+		t.Error("expected fallback when no querier in context")
+	}
+
+	// Inject querier → extract it
+	injected := &Node{name: "injected"}
+	ctx = InjectQuerier(ctx, injected)
+	got = ExtractQuerier(ctx, fallback)
+	if got != injected {
+		t.Error("expected injected querier")
+	}
+}