docs: Add a README explaining the purpose of the benchmark suite

Author: zah

test/benchmarks/README.md

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+This directory contains programs and fixtures used for manual performance testing of the tracer.
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+These benchmarks are **not** executed in CI because they may take longer to run than typical tests. Each program has a reference trace recorded using the current tracer implementation. When we experiment with alternative tracer implementations, we will compare their output with these fixtures to ensure compatibility.
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+The reference traces are stored via Git LFS so the repository stays lightweight. `run_benchmark.rb` verifies the SHA-256 hash of each fixture and downloads it with `git lfs` on demand if missing.
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+At the moment there is a single benchmark (`heavy_work`) that exercises a mixture of array and hash operations while computing prime numbers. More benchmarks will be added as we expand the suite.