feat(ios): 中继通道 P4/P5——端点抽象、信封、传输、配对与分层错误

- InstanceEndpoint 取代 Instance.origin（旧数据 init(from:) 读成 .direct，
  写盘只输出 endpoint；dedupKey 直通用 origin、中继用 relay:<nodeID>）
- RelayWire：Swift 侧重算 contracts/test-vectors.json（CanonicalJSON 手写解析器，
  因为 Foundation 的解码器会抹掉'拼写'，而 intent 摘要定义的正是拼写）
- NodeTransport 抽象 + DirectTransport / RelayTransport / NodeTransportRouter；
  DirectTransport 是薄适配，直通路径行为不变
- RelayTransport：信封构造、签名、加密、验签、解密；响应验签失败即丢弃，
  四类以上失败各有独立 UpstreamError 分支与用户文案
- DeviceIdentity（沙盒文件 0600，配对时由节点分配 deviceID）、RelayPairing、配对 sheet
- 卡片通道徽标 + freshness 副标题；危险写 intent 在发送瞬间生成
- scripts/export-app-routes.swift 生成 vendored 路由表，AppRoutesContractTests 反向守卫
- 测试 78→92：向量互算、迁移（字面量旧 JSON）、配对四类失败、路由器、
  传输往返与各类拒绝路径

apps/ios/Sources/MultiSimAdmin/Features/Messages/MessageService.swift

@@ -7,18 +7,19 @@ public enum MessageServiceError: Error, Equatable {
 }
 
 public struct MessageService: Sendable {
-    private let client: any SimAdminClientProtocol
-    private let sessions: InstanceSessionStore
-    private let credentials: (any CredentialStore)?
+    private let transport: any NodeTransport
 
+    public init(transport: any NodeTransport) {
+        self.transport = transport
+    }
+
+    /// Direct-mode construction, kept for the existing call sites and tests.
     public init(
         client: any SimAdminClientProtocol,
         sessions: InstanceSessionStore,
         credentials: (any CredentialStore)? = nil
     ) {
-        self.client = client
-        self.sessions = sessions
-        self.credentials = credentials
+        self.init(transport: DirectTransport(client: client, sessions: sessions, credentials: credentials))
     }
 
     public func messages(
@@ -75,30 +76,7 @@ public struct MessageService: Sendable {
     }
 
     private func request(_ route: UpstreamRoute, _ instance: Instance) async throws -> UpstreamResponse {
-        var cookie = await sessions.cookie(for: instance.id, origin: instance.origin)
-        do {
-            return try await client.send(route, baseURL: instance.origin, cookie: cookie)
-        } catch UpstreamError.unauthorized, UpstreamError.forbidden {
-            cookie = try await login(instance)
-            return try await client.send(route, baseURL: instance.origin, cookie: cookie)
-        }
-    }
-
-    private func login(_ instance: Instance) async throws -> String {
-        guard let credentials,
-              let password = try credentials.get(instanceID: instance.id),
-              !password.isEmpty
-        else {
-            throw MessageServiceError.unauthorized
-        }
-        let sessions = InstanceSessionService(store: sessions, client: client)
-        let result = try await sessions.login(instance, password: password)
-        guard result.authenticated,
-              let cookie = await sessions.cookie(for: instance)
-        else {
-            throw MessageServiceError.unauthorized
-        }
-        return cookie
+        try await transport.send(route, instanceID: instance.id, endpoint: instance.endpoint)
     }
 
     static func parsePage(_ response: UpstreamResponse, _ instance: Instance) throws -> MessagePage {