fix: use duck-typing for StepConfig detection instead of instanceof

instanceof fails when StepConfig is loaded from different module paths
(e.g., symlinked packages). Add __isStepConfig marker and static
isStepConfig() method for reliable detection across module boundaries.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: DavertMik

lib/els.js

@@ -9,7 +9,7 @@ import { isAsyncFunction, humanizeFunction } from './utils.js'
 
 function element(purpose, locator, fn) {
   let stepConfig
-  if (arguments[arguments.length - 1] instanceof StepConfig) {
+  if (StepConfig.isStepConfig(arguments[arguments.length - 1])) {
     stepConfig = arguments[arguments.length - 1]
   }
 

lib/step/config.js

@@ -16,6 +16,11 @@ class StepConfig {
       timeout: undefined,
       retry: undefined,
     }
+    this.__isStepConfig = true
+  }
+
+  static isStepConfig(obj) {
+    return obj && (obj instanceof StepConfig || obj.__isStepConfig === true)
   }
 
   /**

lib/step/record.js

@@ -11,7 +11,7 @@ function recordStep(step, args) {
 
   // apply step configuration
   const lastArg = args[args.length - 1]
-  if (lastArg instanceof StepConfig) {
+  if (StepConfig.isStepConfig(lastArg)) {
     const stepConfig = args.pop()
     const { opts, timeout, retry } = stepConfig.getConfig()