Publish prebuilt wheels to PyPI (docs, release flow, musllinux)

[prostomarkeloff]

Makes pip install directiva / uv add directiva install a prebuilt abi3 wheel with no Rust toolchain, on every common platform.

What was wrong

The repo shipped abi3 wheels via the CI matrix but documented "No PyPI" — install was a release-asset URL or pip install git+… (the latter builds from source → needs a compiler). Nothing pointed at PyPI.

Changes

• README — install section is now pip install directiva / uv add directiva (prebuilt). Source build kept only as the no-matching-wheel fallback.
• CI — header documents the release procedure (tag → matrix → uv publish dist/*). Added a musllinux_1_2 wheel job (x86_64 + aarch64) so Alpine/musl Docker installs get a prebuilt wheel instead of a source fallback. Added to release.needs.
• Makefile — dist (local one-platform smoke build), dist-fetch TAG= (gh release download), publish (uv publish dist/*).

Verified locally

• uv build → abi3 wheel (cp39-abi3, works on CPython ≥ 3.9) + self-contained sdist.
• Wheel installs into a clean venv with --no-build (compilation forbidden) and imports + runs.

Coverage after this

linux glibc (x86_64/aarch64) · linux musl (x86_64/aarch64) · macOS (x86_64/arm64) · Windows x64 · sdist fallback.

Release (manual, maintainer runs uv publish)

# bump version in Cargo.toml, commit
git tag vX.Y.Z && git push --tags     # CI builds the matrix, attaches to the GH release
make dist-fetch TAG=vX.Y.Z            # pull every wheel + sdist into ./dist
make publish                          # uv publish dist/*  (UV_PUBLISH_TOKEN)

Preconditions: the directiva name is available/owned on PyPI; PyPI token on the maintainer's machine.