Add discussion 602 frontend exam answer

[potter-sun]

Summary

• Adds the answer document for GitHub Discussion [Onboarding Exam] 前端架构基础题 #602 frontend architecture onboarding exam.
• Covers Command / Query(ReadModel) / Observation / local UI state boundaries, ACK semantics, API contract choices, stable UI dependencies, and PR review issues.

Validation

• git diff --cached --check before commit
• Document-only change; build/test not run.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (test/20260511@949d932). Learn more about missing BASE report.

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ci	72.02% <ø> (?)

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