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>

slog.go

@@ -0,0 +1,38 @@
+package dbx
+
+import (
+	"context"
+	"log/slog"
+)
+
+// SlogLogger adapts *slog.Logger to the dbx.Logger interface.
+type SlogLogger struct {
+	Logger *slog.Logger
+}
+
+// NewSlogLogger creates a SlogLogger. If l is nil, slog.Default() is used.
+func NewSlogLogger(l *slog.Logger) *SlogLogger {
+	if l == nil {
+		l = slog.Default()
+	}
+	return &SlogLogger{Logger: l}
+}
+
+func (s *SlogLogger) Debug(ctx context.Context, msg string, fields ...any) {
+	s.Logger.DebugContext(ctx, msg, fields...)
+}
+
+func (s *SlogLogger) Info(ctx context.Context, msg string, fields ...any) {
+	s.Logger.InfoContext(ctx, msg, fields...)
+}
+
+func (s *SlogLogger) Warn(ctx context.Context, msg string, fields ...any) {
+	s.Logger.WarnContext(ctx, msg, fields...)
+}
+
+func (s *SlogLogger) Error(ctx context.Context, msg string, fields ...any) {
+	s.Logger.ErrorContext(ctx, msg, fields...)
+}
+
+// Compile-time check.
+var _ Logger = (*SlogLogger)(nil)