refactor: make UMAP Rust-only

[GeorgePearse]

Summary

• Removes the pure-Python UMAP backend and makes squeeze.UMAP a thin wrapper over the Rust implementation.
• Adds a Rust UMAP implementation (UMAPRust) to the extension module and updates the shipped extension binary.

Notes

• Features that depended on the Python backend (e.g. aligned/composite UMAP helpers and their tests) are currently skipped/guarded until equivalent Rust graph state/ops are exposed.

Testing

• python3 -m pytest squeeze/tests -q

[openhands-ai]

Looks like there are a few issues preventing this PR from being merged!

• GitHub Actions are failing:
  • Performance Benchmarks

If you'd like me to help, just leave a comment, like

@OpenHands please fix the failing actions on PR #12 at branch `refactor/rust-only-umap`

Feel free to include any additional details that might help me get this PR into a better state.

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