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.go

@@ -0,0 +1,28 @@
+package dbx
+
+import (
+	"context"
+
+	"github.com/jackc/pgx/v5"
+)
+
+// Collect executes a read query and collects all rows into a slice of T
+// using pgx.RowToStructByName. T must be a struct with db tags matching column names.
+func Collect[T any](ctx context.Context, c *Cluster, sql string, args ...any) ([]T, error) {
+	rows, err := c.ReadQuery(ctx, sql, args...)
+	if err != nil {
+		return nil, err
+	}
+	return pgx.CollectRows(rows, pgx.RowToStructByName[T])
+}
+
+// CollectOne executes a read query and collects exactly one row into T.
+// Returns pgx.ErrNoRows if no rows are returned.
+func CollectOne[T any](ctx context.Context, c *Cluster, sql string, args ...any) (T, error) {
+	rows, err := c.ReadQuery(ctx, sql, args...)
+	if err != nil {
+		var zero T
+		return zero, err
+	}
+	return pgx.CollectExactlyOneRow(rows, pgx.RowToStructByName[T])
+}