Add optimal execution trading example

Created a new example that demonstrates optimal trading execution:
- Implemented market simulator with price impact model
- Added evaluator with metrics for cost improvement vs baseline
- Fixed parameter ordering in simulate_execution function
- Included timeout handling for search algorithms

Author: timpara

examples/optimal_execution/README.md

@@ -226,7 +226,8 @@ When you add knobs, remember:
 
 5. FAQ  
 ------  
-   
+Q: **How do I run the example?**
+A: Run `python openevolve-run.py examples/optimal_execution/initial_program.py examples/optimal_execution/evaluator.py --iterations 20 --config config.yaml'
 Q: **Why does the evaluator re-implement `simulate_execution`?**    
 A: To guarantee the candidate cannot cheat by hard-coding answers from its own  
 RNG realisations.