Fix workflowsets imports, zero-coefficient labeling, and feature-cutoff bugs from #31 review

[eboyer221]

What

Addresses code review feedback on #31 (the vip::vi() replacement).
Three fixes, plus one item left as an open question:

1. Orphaned imports. @importFrom workflowsets extract_fit_parsnip and extract_spec_parsnip are declared but nothing calls them - .viGlmnet() replaced the old workflowsets |> vip::vi() chain with parsnip::extract_fit_engine() directly. Removed both, and removed workflowsets from DESCRIPTION since nothing in the package uses it anymore.

2. Zero coefficients mislabeled. .viGlmnet()'s Sign = ifelse(coefs > 0, "POS", "NEG") labeled an exact-zero coefficient "NEG" - indistinguishable from an actually-negative one. This matters for LASSO/elastic-net models, where zeroing out a coefficient is the normal way a feature gets excluded. Now labeled NA.

3. Last feature silently dropped. extractTopFeats()'s default prop_vi_top_feats = c(0, 1) is documented to return all features but filter(cum_imp < cum_vi_upper & ...) used a strict <, and the least-important feature's cumulative importance is exactly equal to the total - so it always got excluded. Changed to <=.

Left open

Whether .viGlmnet() should use the tuned penalty (as the multi-class path in extractTopFeats() already does) instead of min(glmnet_fit$lambda) - flagging for @AbhirupaGhosh.

Testing

Added two tests to tests/testthat/test-core-ml.R. I confirmed both catch the original bugs by temporarily reverting the fix and re-running: the zero-coefficient test failed as expected, and testing against a real fitted model reproduced the exact symptom described - the least important feature (feat_noise_1) silently missing from the result. Full suite passes (214/214).