Publish wheels to halide/pypi instead of pypi.halide-lang.org

[alexreinking]

Summary

• Replaces twine upload to pypi.halide-lang.org with gh release create/upload against halide/pypi, tagged <project>@<version> (halide-flatbuffers@..., halide-wabt@...).
• The check job now looks for existing releases via the GitHub API instead of scraping the old PyPI simple-index HTML page.
• Explicitly triggers halide/pypi's rebuild-index.yml after publishing.
• No manifest.json sidecar -- GitHub already computes and exposes a sha256 digest per release asset.

Why

This is the same migration already merged for halide-llvm (halide/halide-llvm#1) and Halide (halide/Halide#9282, pending review): pypi.halide-lang.org now serves halide/pypi (GitHub Releases + Pages) instead of the retired MIT-hosted pypiserver. This repo was actually the first one migrated -- it served as the live test bed for the whole pipeline before touching the other two repos -- but a real PR against main was never opened for it, so main is still on the old twine upload path right now. That path is already broken (there's no pypiserver behind pypi.halide-lang.org anymore); it just hasn't surfaced yet because nobody's pushed a real version bump since the DNS cutover.

Test plan

• Fully exercised for real on this exact branch already: halide-flatbuffers@23.5.26 and halide-wabt@1.0.39 were published to halide/pypi and are live in the generated index right now.
• actionlint passes.