docs(_bert): explain str-key label2id requirement to prevent regression

A future refactor sees `{str(i): i}` as a no-op coercion and "simplifies"
back to `{i: i}`; mypy passes, then BertScorer.fit raises
StrictDataclassFieldValidationError at runtime. Comment makes the WHY
explicit at the call site, matching the WHY-only comment policy from
14f9576.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: voorhs

src/autointent/modules/scoring/_bert.py

@@ -130,6 +130,9 @@ def get_implicit_initialization_params(self) -> dict[str, Any]:
     def _initialize_model(self) -> Any:  # noqa: ANN401
         from transformers import AutoModelForSequenceClassification
 
+        # huggingface_hub v1 StrictDataclass requires label2id keys to be str
+        # (and id2label values to be str); int-keyed dicts raise
+        # StrictDataclassFieldValidationError on from_pretrained in v5.
         label2id = {str(i): i for i in range(self._n_classes)}
         id2label = {i: str(i) for i in range(self._n_classes)}