Add cache size limit support

[shaypal5]

Summary

• implement cache_size_limit and replacement_policy parameters
• default to LRU policy and enforce limit in memory core
• document cache size limiting in README
• test LRU eviction behaviour
• update default params test

Testing

• ruff check .
• mypy src/cachier
• pytest -m "memory or pickle" --cov=cachier --cov-report=term
• ./scripts/test-local.sh all (fails: Docker is not installed)

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https://chatgpt.com/codex/tasks/task_e_687a30b9d6dc8323931ef5c545e0c3bb