docs: add Week 5 design spec (animations & a11y audit)

docs: add Week 5 design spec (animations & a11y audit)

Design covers:
- Subtle, professional entrance animations (fade-in, slide-up, 200-300ms)
- Motion-safe alternatives (prefers-reduced-motion support)
- Focused accessibility audit (focus management, keyboard nav, ARIA basics)
- 60+ test cases covering animations, keyboard navigation, focus indicators
- Success criteria: WCAG 2.1 AA compliance, 60fps performance
- Deliverables: WEEK5-IMPLEMENTATION.md, WEEK5-TESTING.md, A11Y-AUDIT-REPORT.md

Scope: No page transition animations (KISS), no advanced ARIA patterns, focused audit areas only.

Co-Authored-By: Claude Haiku 4.5 <redacted>