Speed up synthesis fitness evaluation

[alcides]

Summary

• Add aeon/synthesis/fitness_eval.py with prefix pre-binding, bundled expression-goal evaluation, and LRU fitness memoization keyed by term hash.
• Wire bundled fitness through EvaluationPool workers so multiple @minimize goals share one interpreter walk on the suffix chain instead of re-evaluating the full program per goal.
• Optimize @property fitness by pre-building call spines and reusing prefix-bound evaluation contexts.

Test plan

• pytest tests/synth_fitness_test.py tests/property_fitness_test.py tests/optimization_decorators_test.py
• Pre-commit on changed files

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