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>

scan_test.go

@@ -0,0 +1,81 @@
+package dbx_test
+
+import (
+	"context"
+	"errors"
+	"testing"
+
+	"git.codelab.vc/pkg/dbx"
+	"git.codelab.vc/pkg/dbx/dbxtest"
+	"github.com/jackc/pgx/v5"
+)
+
+type scanRow struct {
+	ID   int    `db:"id"`
+	Name string `db:"name"`
+}
+
+func TestCollect(t *testing.T) {
+	c := dbxtest.NewTestCluster(t)
+	ctx := context.Background()
+
+	_, err := c.Exec(ctx, `CREATE TEMPORARY TABLE test_collect (id int, name text)`)
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, err = c.Exec(ctx, `INSERT INTO test_collect (id, name) VALUES (1, 'alice'), (2, 'bob')`)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	rows, err := dbx.Collect[scanRow](ctx, c, `SELECT id, name FROM test_collect ORDER BY id`)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(rows) != 2 {
+		t.Fatalf("expected 2 rows, got %d", len(rows))
+	}
+	if rows[0].ID != 1 || rows[0].Name != "alice" {
+		t.Errorf("row 0: got %+v", rows[0])
+	}
+	if rows[1].ID != 2 || rows[1].Name != "bob" {
+		t.Errorf("row 1: got %+v", rows[1])
+	}
+}
+
+func TestCollectOne(t *testing.T) {
+	c := dbxtest.NewTestCluster(t)
+	ctx := context.Background()
+
+	_, err := c.Exec(ctx, `CREATE TEMPORARY TABLE test_collect_one (id int, name text)`)
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, err = c.Exec(ctx, `INSERT INTO test_collect_one (id, name) VALUES (1, 'alice')`)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	row, err := dbx.CollectOne[scanRow](ctx, c, `SELECT id, name FROM test_collect_one WHERE id = 1`)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if row.ID != 1 || row.Name != "alice" {
+		t.Errorf("got %+v", row)
+	}
+}
+
+func TestCollectOneNoRows(t *testing.T) {
+	c := dbxtest.NewTestCluster(t)
+	ctx := context.Background()
+
+	_, err := c.Exec(ctx, `CREATE TEMPORARY TABLE test_collect_norows (id int, name text)`)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	_, err = dbx.CollectOne[scanRow](ctx, c, `SELECT id, name FROM test_collect_norows`)
+	if !errors.Is(err, pgx.ErrNoRows) {
+		t.Errorf("expected pgx.ErrNoRows, got %v", err)
+	}
+}