Refactor BasePlot: avoid unnecessary DataFrame conversion

[etching-7z]

This PR prepares a refactor of the BasePlot implementation.

• Currently, BasePlot converts AnnData objects to a Pandas DataFrame internally,
  which can be inefficient and memory-heavy for large single-cell datasets.
• This branch scaffolds a space for replacing .to_df() usage with direct access
  to AnnData.obs and NumPy arrays where possible.
• Follow-up commits will add the actual refactor and tests.

This is an initial step to discuss approach and confirm direction before proceeding.

• Closes #
• Tests included or not required because:

• Release notes not necessary because: