docs: add migration plan for skipped Choice and TextAreaRegionView files

Analysis of the 2 files initially skipped for Elem "item" overlap.

Key finding: The overlap is not a real CSS conflict!
- Choice.jsx: Block "choice" → generates dm-choice__item
- TextAreaRegionView.jsx: Block "textarea-tag" → generates dm-textarea-tag__item

Different Block names mean different CSS classes, so they're actually safe to migrate.

The conservative skip rule was overly cautious - these files CAN be migrated.

Document includes:
- Detailed analysis of both files
- Migration strategies (together vs sequential)
- Special handling for nameWrapper pattern in Choice
- CSS impact assessment
- Risk assessment
- Implementation notes

Recommendation: Migrate both files to complete the BEM migration effort.

Author: luarmr