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.go

@@ -0,0 +1,42 @@
+package dbx
+
+import (
+	"time"
+
+	"github.com/jackc/pgx/v5/pgxpool"
+)
+
+// PoolStats is an aggregate of pool statistics across all nodes.
+type PoolStats struct {
+	AcquireCount         int64
+	AcquireDuration      time.Duration
+	AcquiredConns        int32
+	CanceledAcquireCount int64
+	ConstructingConns    int32
+	EmptyAcquireCount    int64
+	IdleConns            int32
+	MaxConns             int32
+	TotalConns           int32
+	Nodes                map[string]*pgxpool.Stat
+}
+
+// Stats returns aggregate pool statistics for all nodes in the cluster.
+func (c *Cluster) Stats() PoolStats {
+	ps := PoolStats{
+		Nodes: make(map[string]*pgxpool.Stat, len(c.all)),
+	}
+	for _, n := range c.all {
+		s := n.pool.Stat()
+		ps.Nodes[n.name] = s
+		ps.AcquireCount += s.AcquireCount()
+		ps.AcquireDuration += s.AcquireDuration()
+		ps.AcquiredConns += s.AcquiredConns()
+		ps.CanceledAcquireCount += s.CanceledAcquireCount()
+		ps.ConstructingConns += s.ConstructingConns()
+		ps.EmptyAcquireCount += s.EmptyAcquireCount()
+		ps.IdleConns += s.IdleConns()
+		ps.MaxConns += s.MaxConns()
+		ps.TotalConns += s.TotalConns()
+	}
+	return ps
+}