[codex] Add Rust retrieval prototype benchmark

[brodieacc]

What changed

• add an isolated Rust retrieval prototype under prototypes/rust_search/
• add a Python ctypes wrapper for the prototype under benchmarks/rust_search.py
• add benchmarks/prototype_retrieval.py to compare scalar search, prepared NumPy search, and the Rust prototype
• document the prototype in benchmarks/README.md

Why

This PR is the measurement step: it answers whether native retrieval kernels are worth pursuing before any production API or packaging changes land. The code stays deliberately outside the public library runtime.

Validation

• uv run python -m benchmarks.prototype_retrieval --model MAP --smoke --build-rust

Stack

This PR is stacked on codex/retrieval-semantics-fixes.