#213 Replay: Add exception stacktrace to step_failed events

## Problem The event_logger.log_step_failed() only stores error_message string, not full Python traceback. ## Location engine/utils/event_logger.py:322-370 ## Current Code ```python payload: dict[str, Any] = { "step_name": step_name, "error": error_message, # String only, no traceback } ``` ## Impact - Cannot diagnose divergences from error message alone - Debugging replay failures nearly impossible without stacktrace - Production issues hard to root-cause ## Solution 1. Add `error_traceback` field to step_failed event payload 2. Use `traceback.format_exc()` to capture full stack 3. Update ReplayContext to display stacktrace in simulation reports ## Effort: Small