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>

cluster.go

@@ -17,12 +17,13 @@ type Cluster struct {
 	replicas []*Node
 	all      []*Node // master + replicas for health checker
 
-	balancer Balancer
-	retrier  *retrier
-	health   *healthChecker
-	logger   Logger
-	metrics  *MetricsHook
-	closed   atomic.Bool
+	balancer           Balancer
+	retrier            *retrier
+	health             *healthChecker
+	logger             Logger
+	metrics            *MetricsHook
+	slowQueryThreshold time.Duration
+	closed             atomic.Bool
 }
 
 // NewCluster creates a Cluster, connecting to all configured nodes.
@@ -53,13 +54,14 @@ func NewCluster(ctx context.Context, cfg Config) (*Cluster, error) {
 	all = append(all, replicas...)
 
 	c := &Cluster{
-		master:   master,
-		replicas: replicas,
-		all:      all,
-		balancer: NewRoundRobinBalancer(),
-		retrier:  newRetrier(cfg.Retry, cfg.Logger, cfg.Metrics),
-		logger:   cfg.Logger,
-		metrics:  cfg.Metrics,
+		master:             master,
+		replicas:           replicas,
+		all:                all,
+		balancer:           NewRoundRobinBalancer(),
+		retrier:            newRetrier(cfg.Retry, cfg.Logger, cfg.Metrics),
+		logger:             cfg.Logger,
+		metrics:            cfg.Metrics,
+		slowQueryThreshold: cfg.SlowQueryThreshold,
 	}
 
 	c.health = newHealthChecker(all, cfg.HealthCheck, cfg.Logger, cfg.Metrics)
@@ -232,6 +234,13 @@ func (c *Cluster) queryEnd(ctx context.Context, node, sql string, err error, d t
 	if c.metrics != nil && c.metrics.OnQueryEnd != nil {
 		c.metrics.OnQueryEnd(ctx, node, sql, err, d)
 	}
+	if c.slowQueryThreshold > 0 && d >= c.slowQueryThreshold {
+		c.logger.Warn(ctx, "dbx: slow query",
+			"node", node,
+			"duration", d,
+			"sql", sql,
+		)
+	}
 }
 
 // errRow implements pgx.Row for error cases.