Include case names in multi-index legends

[ronangiss]

Summary

• Route non-aligned plot_data legend fallback through the existing case/variable legend helper.
• Preserve current single-index and single-selection labeling while adding case - variable labels when both obj.selection and obj.inds contain multiple entries.

Testing

• git show --check --stat --oneline HEAD
• Static legend label routing check
• MATLAB block-balance check for compareWingbeatUI.m

Notes

• MATLAB/Octave is not installed in this cloud environment, so runtime GUI validation is limited to static validation here.