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/meta.go

@@ -0,0 +1,42 @@
+package gateway
+
+import (
+	"net/http"
+
+	"git.yeij.top/AskaEth/Cyrene_v2/internal/contract/enums"
+)
+
+// This file owns the read-only client metadata surface: GET /api/v1/meta/enums,
+// which publishes the authoritative platform/mood/mode tables (Android
+// collaboration B4, design/07 §6). Clients (Web, Android, an IM bridge) read the
+// tables instead of hardcoding their own id → label mapping, so a source badge or
+// a mood label cannot drift per client. The tables themselves live in
+// internal/contract/enums; this handler only serves them, so there is one source
+// of truth rather than a copy per consumer.
+
+// metaEnums is the GET /api/v1/meta/enums payload. `unknown` carries the
+// machine-readable fallback policy (enums.UnknownFallback) so a client reads the
+// tables and the rule for an unlisted value from the same response.
+type metaEnums struct {
+	Platforms []enums.Item `json:"platforms"`
+	Moods     []enums.Item `json:"moods"`
+	Modes     []enums.Item `json:"modes"`
+	Unknown   string       `json:"unknown"`
+}
+
+// handleListEnums serves the shared client enum tables. It is a pure projection
+// of the Go-side authority (no store, no error path) and contains nothing
+// sensitive, so it is gated like GET /api/v1/sessions: any authenticated role may
+// read it.
+//
+// Adding a value to a table is a non-breaking change for existing clients, which
+// must display an unlisted value raw (see enums.UnknownFallback) rather than
+// dropping it or erroring.
+func (s *Server) handleListEnums(w http.ResponseWriter, _ *http.Request) {
+	writeJSON(w, http.StatusOK, metaEnums{
+		Platforms: enums.Platforms(),
+		Moods:     enums.Moods(),
+		Modes:     enums.Modes(),
+		Unknown:   enums.UnknownFallback,
+	})
+}