feat: add handoff journal route with carry-forward notes

[iamasx]

Summary

• add a standalone handoff-journal App Router route with route-local handoff entry, action bullet, and carry-forward data
• build responsive handoff entry cards plus a compact carry-forward rail using route-specific styling
• add render tests that cover the hero, continuity stats, handoff entries, and carry-forward notes
• add the new route to the landing page index so the journal is discoverable from /

Testing

• npm run lint
• npx -y node@22.12.0 node_modules/vitest/vitest.mjs run src/app/handoff-journal/page.test.tsx
• npx -y node@22.12.0 node_modules/next/dist/bin/next build

Notes

• full Vitest suite still has unrelated baseline failures in src/app/field-guide/page.test.tsx and src/app/scenario-board/page.test.tsx
• local verification used node@22.12.0 via npx because the session default node v20.15.0 is below several toolchain engine requirements

Closes #174