feat: add purposeful interactive design

[Gnaroshi]

Summary

• add progressive cross-document View Transitions, native scroll progress, and reduced-motion-safe state transitions
• replace generic Research question tabs and repeated dividers with meaningful sticky section indexes and structured assessment surfaces
• clarify Projects and project-detail hierarchy, evidence interactions, and numbered workflow causality
• keep active horizontal navigation fully visible across EN/KO mobile routes

Design constraints

• no animation framework, looping effects, parallax, or custom cursor
• content and controls remain complete without experimental browser support
• private source and public content-feed boundaries are unchanged

Verification

• npm run content:check
• npm run check
• npm run build
• npm run check:i18n
• npm run check:public-tone
• npm run check:launch-content
• npm run check:navigation-consistency
• npm run check:links
• npm run media:check
• npm run check:project-readiness
• npm run check:project-copy
• npm run test:feed-contract (11 valid builds, 6 rejection fixtures)
• npm run test:e2e (177 passed, 5 fixture-only skipped)
• npm run test:a11y (55 passed)
• npm run test:performance (7 passed, 0 hydrated islands on audited routes)
• npm run test:visual (52 passed)
• npm run check:interactive-controls (24 passed)