feat(contract,gateway): B4 枚举权威表 — GET /api/v1/meta/enums + 未知值回落约定

- 新增 internal/contract/enums 叶子包：platform/mood/mode 三表 + UnknownFallback="display-raw"；每次调用返回新切片，无包级可变全局
- GET /api/v1/meta/enums（anyAuth 只读、纯投影、无 store/无错误路径）
- 反漂移 4 处：真构造内置适配器断言 Name() 与表双向相等、真工厂 buildAccountAdapter 逐项可建、mood 对齐 emotion 常量 + 默认转移表、mode 对齐前端 MODES（跨语言，文件缺失则 skip）
- Docs/api-client §1.1 枚举权威表 + 行15 + v0.38；api-reference §3.7 + 行12

internal/services/gateway/routes.go

@@ -58,6 +58,12 @@ func (s *Server) registerUserRoutes(
 	mux.Handle("POST /api/v1/chat", writeAuth(http.HandlerFunc(s.handleChat)))
 	mux.Handle("GET /api/v1/sessions", anyAuth(http.HandlerFunc(s.handleListSessions)))
 	mux.Handle("GET /api/v1/personas", anyAuth(http.HandlerFunc(s.handleListPersonas)))
+	// Shared client enum tables (Android collaboration B4, design/07 §6): the
+	// authority every client maps ids to labels from, so a source badge or a mood
+	// label cannot drift per client. A read-only projection of
+	// internal/contract/enums with nothing sensitive in it, so any authenticated
+	// role may read it (same gate as /sessions above).
+	mux.Handle("GET /api/v1/meta/enums", anyAuth(http.HandlerFunc(s.handleListEnums)))
 
 	mux.Handle("POST /api/v1/audio/transcribe", ownerAuth(http.HandlerFunc(s.handleTranscribe)))
 	mux.Handle("POST /api/v1/audio/speech", ownerAuth(http.HandlerFunc(s.handleSpeech)))