Docs/engineering guides

[herefindalex]

Summary

• Add multilingual engineering guides in English, Traditional Chinese, and Simplified Chinese.
• Document system architecture, request and data flows, persistence boundaries, authentication, operations, backup and recovery, and production-evidence limits.
• Add an engineering case study covering six implementation decisions, alternatives, trade-offs, supporting evidence, and evolution triggers.
• Reframe the three top-level READMEs around implemented scope and direct navigation to the engineering guides.
• Restore the three-language “Why Prods” value-proposition sections.
• Keep internal requirements and ADR material private while linking explanations to public source and tests.

Documentation set

• docs/en/{architecture,engineering-case-study,operations}.md
• docs/zh-TW/{architecture,engineering-case-study,operations}.md
• docs/zh-CN/{architecture,engineering-case-study,operations}.md
• README.md
• README-zh_TW.md
• README-zh_CN.md

Verification

• The branch contains exactly the intended 12 documentation files and no source-code changes.
• All relative Markdown links resolve.
• English, Traditional Chinese, and Simplified Chinese guide structures are aligned.
• Version and review-date claims match VERSION (v0.6.8) and the reviewed revision.
• git diff --check passes.
• No internal document content is included.

Before merge

• Restore the original product-positioning and target-audience paragraphs that were not moved into the guides.
• Align the Why Prods platform wording across all three languages, including the macOS cross-build qualification.
• Fix the duplicated “站点站点语言” wording in README-zh_CN.md.

[thomas-chu-30]

approval

[thomas-chu-30]

Approval !!