Build Linux wheels across Python and libc matrix

[bright2227]

Summary

• build CPython 3.12 and 3.13 wheels for manylinux x86-64 and musllinux x86-64
• exercise the distribution workflow on pull requests and manual runs
• retain PyPI upload as a release-only job
• validate each installed wheel imports both native extensions
• inspect both extension ELF dynamic sections and reject any liburing.so dependency
• upload the sdist and each wheel as separate workflow artifacts

Why

PR #17 makes source builds self-contained, but the release workflow currently publishes only an sdist. Without compatible wheels, users still need a compiler and make. This matrix establishes the first binary Linux release targets and verifies that the vendored static-linking guarantee holds in both glibc and musl build containers.

This PR is stacked on #17 (feature/self-contained-liburing-build). PR #17 itself is unchanged.

Validation

• .venv/bin/pytest — 25 passed
• Ruff check and format check pass for scripts/verify_wheel.py
• workflow parses as valid YAML
• local verifier imports _liburing and _uringcore_liburing and finds no liburing.so
• the four container builds are intentionally left to this draft PR's GitHub Actions matrix