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>

dbxtest/tx.go

@@ -0,0 +1,27 @@
+package dbxtest
+
+import (
+	"context"
+	"testing"
+
+	"git.codelab.vc/pkg/dbx"
+)
+
+// RunInTx executes fn inside a transaction that is always rolled back.
+// This is useful for tests that modify data but should not leave side effects.
+// The callback receives a dbx.Querier (not pgx.Tx) so it is compatible with
+// InjectQuerier/ExtractQuerier patterns.
+func RunInTx(t testing.TB, c *dbx.Cluster, fn func(ctx context.Context, q dbx.Querier)) {
+	t.Helper()
+
+	ctx := context.Background()
+	tx, err := c.Begin(ctx)
+	if err != nil {
+		t.Fatalf("dbxtest.RunInTx: begin: %v", err)
+	}
+	defer func() {
+		_ = tx.Rollback(ctx)
+	}()
+
+	fn(ctx, tx)
+}