feat: add PLSCAN clustering and backbone reducer

[GeorgePearse]

Summary

Adds Rust implementations of PLSCAN clustering and a PLSCANBackbone dimensionality reducer, exposed via the PyO3 backend.

Notes

This PR is stacked on top of the Rust UMAP backend PR and targets that branch as its base.

Changes

• Add src/plscan.rs (PLSCAN clustering).
• Add src/plscan_backbone.rs (landmark + spectral reducer backed by PLSCAN).
• Export both classes from src/lib.rs.
• Expose PLSCAN/PLSCANBackbone at the top-level API and add a plscan_backbone strategy.
• Add basic shape/sanity tests (skip if backend not built).

Testing

• Not run.