Vectorize Rosenbrock Hessian and add benchmarks

[farukalpay]

Summary

• Vectorize rosenbrock_hess to remove Python-level loops
• Add pytest-benchmark microbenchmark with tracemalloc memory check
• Document profiling results and performance table in README

Testing

• pre-commit run --files src/psd/functions.py benchmarks/test_rosenbrock_hess.py benchmarks/profiling_notes.md benchmarks/rosenbrock_hess_baseline.json README.md
• PYTHONPATH=src pytest

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https://chatgpt.com/codex/tasks/task_e_68aaf6e83ba88323a58de389f3aefe48