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>

options.go

@@ -1,5 +1,11 @@
 package dbx
 
+import (
+	"time"
+
+	"github.com/jackc/pgx/v5"
+)
+
 // Option is a functional option for NewCluster.
 type Option func(*Config)
 
@@ -31,6 +37,25 @@ func WithHealthCheck(h HealthCheckConfig) Option {
 	}
 }
 
+// WithSlowQueryThreshold sets the threshold for slow query warnings.
+// Queries taking longer than d will be logged at Warn level.
+func WithSlowQueryThreshold(d time.Duration) Option {
+	return func(c *Config) {
+		c.SlowQueryThreshold = d
+	}
+}
+
+// WithTracer sets the pgx.QueryTracer on master and all replica configs.
+// This enables OpenTelemetry integration via libraries like otelpgx.
+func WithTracer(t pgx.QueryTracer) Option {
+	return func(c *Config) {
+		c.Master.Tracer = t
+		for i := range c.Replicas {
+			c.Replicas[i].Tracer = t
+		}
+	}
+}
+
 // ApplyOptions applies functional options to a Config.
 func ApplyOptions(cfg *Config, opts ...Option) {
 	for _, o := range opts {