docs(readme): complete setup and judge demo

[EcstaceeLOR]

Summary

• expand the README with plain-language product behavior, all five Zcash RPC methods, clean-clone setup, deployment, privacy, and MVP limitations
• document the routed Testnet customer-to-merchant flow against the latest merged checkout and payment-verifier behavior
• add a timed judge walkthrough plus screenshot, evidence, and secret-redaction checklists

Verification

• prettier --check README.md docs/judge-demo.md
• npm run lint
• npm run typecheck
• npm test -- --pool=threads --maxWorkers=1 --no-file-parallelism (129 passed, 1 credential-dependent live test skipped)
• npm run secret:check
• npm run build
• all repository-relative Markdown links resolve

The default fork pool hit Windows worker-start timeouts on this host; the unchanged suite passed in a single worker thread.

Closes #13

[Alike001]

Lead review complete. I corrected the judge walkthrough so its claims match the implemented UI: landing-page network rail vs. create-page RPC evidence, Testnet TAZ display, and the validateaddress boundary. Verified formatting, lint, types, production build, secret scan, 129 tests, and the live five-method RPC contract. Ready to squash merge.