[codex] Add optional Rust retrieval backend and release path

[brodieacc]

What changed

• add opt-in search_backend="rust" support for ItemStore and AssocStore on the retrieval modes that benchmarked well
• add a public comparison example and performance docs for the native backend
• vendor the native library into wheels and prefer the packaged _native path at runtime
• add wheel packaging via setup.py/MANIFEST.in
• switch the release path to cibuildwheel, including a real manylinux build
• add a reusable installed-wheel smoke test and a manual TestPyPI publish-and-verify workflow

Why

The prototype showed a clear win for discrete and sparse retrieval kernels. This PR turns that into a shippable, opt-in backend with an actual distribution path, while keeping NumPy as the default and fallback behavior.

Validation

• uv run --extra dev pytest --no-cov
• uv run --extra dev mypy holovec
• uv run --extra dev ruff check holovec tests benchmarks setup.py scripts
• CIBW_PLATFORM=linux uvx cibuildwheel --output-dir wheelhouse
• uvx twine check wheelhouse/*
• HOLOVEC_BUILD_RUST=1 uv build --wheel
• HOLOVEC_SKIP_RUST_BUILD=1 uv build --sdist
• clean wheel-install smoke via python scripts/release_smoke_rust_backend.py

Stack

This PR is stacked on codex/rust-retrieval-prototype.