Epic: PyPI wheel support in cx-wasm (repodata v3 / conda-pypi)

[jezdez]

What?

Enable conda install <package> in the browser to install pure Python wheels from PyPI, extending cx-wasm beyond emscripten-forge packages. This uses the conda-pypi plugin and the upcoming repodata v3 format.

Why?

emscripten-forge has a limited package set. Many pure Python packages (requests, click, etc.) are available as noarch wheels on PyPI. The conda-pypi-test channel indexes ~500K pure Python wheels. With v3 repodata support, the solver can resolve these alongside conda packages.

User impact

• %conda install requests works in the browser without needing the package on emscripten-forge
• Massive expansion of available packages in JupyterLite notebooks

Goals

• cx-wasm solver handles v3.whl repodata records
• Wheel extraction works in MEMFS via conda-pypi extractor
• End-to-end %conda install <pypi-package> in JupyterLite

Tasks

• Parse v3.whl records in sharded.rs (parse_repodata_text currently ignores experimental_v3)
• Handle absolute download URLs from WhlPackageRecord entries (pointing to files.pythonhosted.org)
• Address repodata size (~500K packages in repodata.json — need .zst compression or subsetting)
• Include conda-pypi extractor and installer library in the JupyterLite environment
• Disable conda-pypi post_command hooks in WASM (subprocess-dependent, would cause confusing errors)

This epic is blocked by:

• conda-pypi release (conda/conda-pypi#277)
• conda-pypi-test PR #19 (v3 repodata generation)

References

• cx-wasm already uses rattler_conda_types v0.43.5 which supports v3.whl deserialization
• Relevant CEPs: CEP 111, CEP 145, CEP 146
• See PLAN.md §6 "Future: PyPI wheel support via conda-pypi" for full analysis

[jezdez]

Moved to jezdez/conda-wasm#3 because browser/WASM PyPI wheel support now belongs in the conda-wasm repository.