Add email translator test documentation

[nonggde]

Closes #498

Summary

• add a contributor-facing Email Translator test plan and review notes
• add synthetic review scenarios covering ready, source-review, unsupported-language, and blocked-input states
• add a zero-dependency Node documentation/fixture contract test

Scope

• only touches new docs, fixture, and test files under tools/v2/individual/email-translator/
• intentionally avoids README/specs and existing core/security paths to reduce conflicts with open PRs
• no app shell, routing, auth, wallet, Stellar, database, inbox, mail rendering, live provider, LLM, or design-system integration
• no live network calls, secrets, production data, persistence, or side effects

Validation

• node --test tools/v2/individual/email-translator/tests/email-translator-docs.test.mjs
• npx --yes prettier --check on the new Email Translator docs, fixture, and test files
• git diff --check
• ASCII scan over the changed docs, fixtures, and tests