[codex] Add optional ESM embedding helpers

[Axfff]

Summary

• Add pypropel.esm helpers for optional ESM model loading, sequence embedding, batch embedding, metadata, and NPZ save/load workflows.
• Expose the ESM helper module through the public package namespace.
• Add optional Poetry/setuptools extras for torch and fair-esm.
• Add documentation for ESM embedding usage.
• Add tests using fake torch/esm dependencies so the optional heavy packages are not required for the test suite.

Validation

• /Users/zhaoj/Project/TMInter/.venv/bin/python -m unittest discover -s tests
• ESM worktree result: Ran 6 tests ... OK

Created as a draft for maintainer review.