Cancel pending futures on target-score or early-stop termination

[jink-ucla]

Summary

This PR makes ProcessParallelController.run_evolution() stop work faster when termination conditions are already met.

Problem

When a run reaches target_score or triggers early stopping, pending futures may continue running unnecessarily until natural completion. This
wastes compute and delays clean shutdown.

Changes

• Add explicit termination_reason tracking in run_evolution.
• Set termination reason when:
  • target score is reached
  • early stopping triggers
  • shutdown is requested
• Cancel all remaining pending futures immediately for early-termination paths.
• Improve completion logging so it reports the actual stop reason:
  • target_score
  • early_stopping
  • shutdown
  • no_pending_futures
  • max_iterations
• Add focused unit test:
  • test_target_score_fast_exit_cancels_pending_futures

Why this is safe

• No change to scoring, sampling, or island logic.
• Behavior only changes after a termination condition is already satisfied.
• Cancellation is best-effort and only affects remaining pending work.

Validation

python -m py_compile openevolve/process_parallel.py tests/test_process_parallel.py
python -m unittest tests.test_process_parallel.TestProcessParallel.test_target_score_fast_exit_cancels_pending_futures -v

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