docs: freeze YuXinGu lexicon, journey, and P1 engineering sources

Establish product language contract, user journey with dual growth engines,
ERD/OpenAPI/Go boundaries, and H5 routes (/profile, /portrait, /relation,
/membership) before P1 implementation.

Co-authored-by: Cursor <cursoragent@cursor.com>

.ai/design/design-system.md

@@ -248,7 +248,8 @@ Contrast for body text; touch ≥ 44px; no hover-only essentials; label interact
 ## 13. Brand Voice
 
 温和、清晰、尊重。禁止恐吓 / 道德绑架 / 虚假倒计时.  
-Report/Ask: lifestyle disclaimer（非医疗、非算命）. Labels follow `.ai/domain.md`.
+成长报告 / 问答：生活建议向免责声明（非医疗、非占卜）.  
+**中文命名强制：** `.ai/product/lexicon.md`（无「解码/运势/合盘/解锁」等禁用主文案）.
 
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