feat(gateway,loop): A1 流式帧附加 sessionId/turnId — 多会话并发流式可路由

- model.StreamEvent 增 SessionID/TurnID（适配器在 loop 之下，恒留零值）
- loop 在回合边界注入；覆盖 stream.go 与 tools.go 的 tool_execution 发射点
- 网关 runTurn 每回合生成 turnID 并经 loop.WithTurnID 挂进运行上下文（编排层与 legacy 直连的唯一收口点）；broadcastStream 填充 wsEvent
- Docs/api-client v0.36 契约与变更日志；v0.10 条目标注为已被补齐
- 新增 loop/gateway 测试，含变异验证

internal/services/gateway/ws_hub_turn.go

@@ -80,6 +80,12 @@ func (h *hub) streamChat(ctx context.Context, payload any) {
 // emits a single whole-text chunk to hold the stream contract.
 func (h *hub) runTurn(ctx context.Context, sessionID string, msgs []model.Message, env loop.SessionEnv) (*loop.Result, error) {
 	turnID := newTurnID()
+	// Thread the turn identity into the run context so the loop stamps it on every
+	// streamed event it emits (message.*/thinking.*/tool_call.*); the hub then puts
+	// it on the outbound frame. Every turn takes a fresh id — a session has at most
+	// one in-flight turn — so a client can correlate one burst of deltas to one
+	// assistant turn (design/16 §4.2).
+	ctx = loop.WithTurnID(ctx, turnID)
 	if h.runner == nil || len(msgs) == 0 {
 		h.broadcast(wsEvent{Type: "stream_end", TurnID: turnID, Seq: int(h.streamSeq.Add(1)), SessionID: sessionID, Data: map[string]any{"stopReason": "idle"}})
 		return &loop.Result{StopReason: "idle"}, nil
@@ -117,16 +123,17 @@ func (h *hub) handleInterrupt(payload json.RawMessage) {
 // broadcastStream maps a streamed model.StreamEvent to its downstream WS event
 // type and fans it out to every client. It runs on the bus subscription worker;
 // seq is assigned atomically so a burst stays ordered even if the loop emits
-// events from more than one goroutine. TurnId is left empty here because the
-// OnStream bus payload does not carry the turn identity (future work would
-// thread a turn id through loop → streamChunk → payload).
+// events from more than one goroutine. SessionID/TurnID ride on the event itself
+// (the loop stamps them at the turn boundary from the run context), so the frame
+// carries the scope a client needs to route it to the active session and to one
+// assistant turn.
 func (h *hub) broadcastStream(payload any) {
 	ev, ok := payload.(model.StreamEvent)
 	if !ok {
 		return
 	}
 	typ, data := streamEventData(ev)
-	h.broadcast(wsEvent{Type: typ, Seq: int(h.streamSeq.Add(1)), Data: data})
+	h.broadcast(wsEvent{Type: typ, SessionID: ev.SessionID, TurnID: ev.TurnID, Seq: int(h.streamSeq.Add(1)), Data: data})
 }
 
 // chatStreamRequest is the decoded ws.chat payload.