Add production features: slog adapter, scan helpers, slow query logging, pool stats, tracer passthrough, test tx isolation

- slog.go: SlogLogger adapts *slog.Logger to dbx.Logger interface
- scan.go: Collect[T] and CollectOne[T] generic helpers using pgx.RowToStructByName
- cluster.go: slow query logging via Config.SlowQueryThreshold (Warn level in queryEnd)
- stats.go: PoolStats with Cluster.Stats() aggregating pool stats across all nodes
- config.go/node.go: NodeConfig.Tracer passthrough for pgx.QueryTracer (OpenTelemetry)
- options.go: WithSlowQueryThreshold and WithTracer functional options
- dbxtest/tx.go: RunInTx runs callback in always-rolled-back transaction for test isolation

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

stats_test.go

@@ -0,0 +1,22 @@
+package dbx_test
+
+import (
+	"testing"
+
+	"git.codelab.vc/pkg/dbx/dbxtest"
+)
+
+func TestStats(t *testing.T) {
+	c := dbxtest.NewTestCluster(t)
+
+	ps := c.Stats()
+	if ps.Nodes == nil {
+		t.Fatal("Nodes map is nil")
+	}
+	if _, ok := ps.Nodes["test-master"]; !ok {
+		t.Error("expected test-master in Nodes")
+	}
+	if ps.MaxConns <= 0 {
+		t.Errorf("expected MaxConns > 0, got %d", ps.MaxConns)
+	}
+}