Refactor FDE demo into a failure testing workbench

[echowang1]

Summary

• rebuild the demo around a persistent presentation rail and a focused failure-test stage
• add clear Chinese product positioning, target user, inputs, outputs, and V0.1 boundaries
• replace production payment language with fixture testing, policy result, reason code, and test receipt semantics
• update the README and FDE playbook to match the new presentation flow

Verification

• npm run typecheck
• npm test (17 files, 44 tests)
• npm run build
• node/CSS/HTML contract checks
• JSDOM interaction smoke test across fail, pass, review, receipt tabs, rule trace, and fixture editing