chore: declare matplotlib and apply three behavior-preserving nits

- pyproject.toml: add a figures extra with matplotlib (used by the two figure
  scripts) so the environment is reproducible from pyproject alone.
- partition.py: correct the partition_eval_split docstring (simulated per-client
  eval sets from the global split, not client-owned held-out data).
- protean_cnn.py: extract_embeddings now moves batches to the model device.
- posthoc_statistical_audit.py: refuse to write an empty table.

pyproject.toml

@@ -20,6 +20,10 @@ research = [
     "pandas>=2.0",
     "torch>=2.0",
 ]
+figures = [
+    # Used by scripts/make_final_figures.py and scripts/make_comparison_figures.py.
+    "matplotlib>=3.8",
+]
 dev = [
     "pytest>=7.0",
 ]