Add AlchemBERT results and fix classification scoring compatibility

[liuxiaotong15]

Benchmark submissions

Brief description of your algorithm

We test BERT on all tasks of Matbench.

Included files

-- benchmarks
---- matbench_v0.1_AlchemBERT
------ results.json.gz
------ readme.py              # please visit our repo to get all codes
------ info.json                 

[liuxiaotong15]

A note on the scope of this PR:

This PR was originally opened as an AlchemBERT benchmark submission. While
bringing the branch up to date and getting the submission checks to pass, it
also became necessary to include several compatibility and scoring fixes:

1. score_array() now preserves the original probability predictions for
   ROC-AUC while using separately thresholded labels for accuracy, balanced
   accuracy, and F1. This fixes the issue described in [MUST FIX] How does ROCAUC work in score_array()? #137 and follows up on
   the classification probability discussion in Error when recording group probability results to classification tasks #40.

2. The classification helpers and tests now support NumPy scalar types and
   NumPy 2. The removed np.float_ alias was replaced by dtype-category checks
   using np.issubdtype.

3. The coverage artifact workflow now uses
   actions/upload-artifact@v4 and actions/download-artifact@v4, because
   GitHub rejects the deprecated v3 actions.

I considered splitting the benchmark submission and core compatibility fixes
into separate PRs. However, the original standalone AlchemBERT branch no
longer passes against the repository's current dependency and GitHub Actions
environment. I have therefore kept the changes together in this PR so the
submission remains reproducible and all required checks can pass.

Current checks:

• test (3.9): passed
• benchmark_submission: passed
• code_coverage: passed

The ROC-AUC change affects future scoring and does not rewrite historical
leaderboard artifacts.