feat: add Rust UMAP backend

[GeorgePearse]

Summary

Adds a minimal Rust UMAP implementation to the PyO3 backend and exposes it as squeeze.RustUMAP.

Changes

• Add Rust UMAP implementation in src/umap.rs and export it from src/lib.rs.
• Expose RustUMAP in squeeze/init.py and register an umap_rust strategy.
• Make Rust backend symbol imports resilient (missing symbols do not disable the whole backend).
• Add a small smoke test (skips if backend not built).

Testing

• Not run (requires rebuilding the Rust extension to exercise RustUMAP).

[openhands-ai]

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

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

@OpenHands please fix the failing actions on PR #9 at branch `feat/rust-umap-backend`

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

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