Fix UMAP crashes: upgrade annembed and add panic safety

[cpetersen]

Summary

• Upgrade annembed dependency from clusterkit-0.1.1 to clusterkit-0.2.6, which fixes multiple UMAP embedding crashes (see scientist-labs/annembed#9)
• Wrap embedder.embed() in catch_unwind so any remaining Rust panics become Ruby RuntimeError exceptions instead of killing the process

Bugs fixed in annembed clusterkit-0.2.6

1. LAPACK SGESDD parameter 12 crash — SVD received wrong matrix layout (Fortran data declared as C-order)
2. ndarray index-out-of-bounds in diffusion maps — off-by-one when accessing eigenvectors/eigenvalues with [j+1] offset
3. Dimension mismatch in entropy optimization — asked_dim used to read vectors that could have fewer columns
4. Empty neighbor array panics — unguarded neighbours[0] access

Test plan

• Rust tests pass: cargo test --no-default-features --features macos-accelerate (2 passed)
• Ruby specs pass: bundle exec rspec (465 passed, 0 failures)
• End-to-end: ragnar umap train with 6938 embeddings (768 dims) completes successfully
• Previously crashed with both LAPACK fatal error and index-out-of-bounds panic

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