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_test.go

@@ -0,0 +1,52 @@
+package dbx
+
+import (
+	"bytes"
+	"context"
+	"log/slog"
+	"strings"
+	"testing"
+)
+
+func TestSlogLogger(t *testing.T) {
+	var buf bytes.Buffer
+	h := slog.NewTextHandler(&buf, &slog.HandlerOptions{Level: slog.LevelDebug})
+	l := NewSlogLogger(slog.New(h))
+
+	ctx := context.Background()
+
+	l.Debug(ctx, "debug msg", "key", "val1")
+	l.Info(ctx, "info msg", "key", "val2")
+	l.Warn(ctx, "warn msg", "key", "val3")
+	l.Error(ctx, "error msg", "key", "val4")
+
+	out := buf.String()
+
+	for _, want := range []string{
+		"level=DEBUG",
+		"debug msg",
+		"key=val1",
+		"level=INFO",
+		"info msg",
+		"key=val2",
+		"level=WARN",
+		"warn msg",
+		"key=val3",
+		"level=ERROR",
+		"error msg",
+		"key=val4",
+	} {
+		if !strings.Contains(out, want) {
+			t.Errorf("output missing %q\ngot: %s", want, out)
+		}
+	}
+}
+
+func TestNewSlogLoggerNil(t *testing.T) {
+	l := NewSlogLogger(nil)
+	if l.Logger == nil {
+		t.Fatal("expected non-nil logger when passing nil")
+	}
+	// should not panic
+	l.Info(context.Background(), "test")
+}