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/ai-contract.md

@@ -10,7 +10,7 @@ MUST read:
 2. `.ai/architecture.md`
 3. `.ai/domain.md` (+ `domain/domain-map.md` when touching models/API)
 4. `.ai/coding.md`
-5. Task-relevant: `api.md` / `database.md` / `ui.md` / `security.md` / `product/feature-map.md`
+5. Task-relevant: `api.md` / `database.md` / `ui.md` / `security.md` / `product/lexicon.md` + `product/feature-map.md`
 6. UI tasks: `design/design-system.md` + `design/component-catalog.md` + `design/platform/*`
 7. Relevant ADR under `.ai/adr/` if changing stack or style