docs: reorganize into design/, standards/, test/ subdirectories

Move docs into topic-based subdirectories, remove stale files
(IOS_PARITY_PLAN, RECENT_WORK_CONTEXT, UI_UX_ELEVATION_PLAN),
and add ARCHITECTURE.md.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: alichherawalla