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/extra_test.go

@@ -948,8 +948,10 @@ func TestHubBroadcastStream(t *testing.T) {
 
 	// A text_delta StreamEvent on the OnStream→bus→relay path becomes a typed
 	// message.delta frame carrying the delta text and a monotonically increasing
-	// seq, leaving the legacy stream_chunk shape out of the way.
-	hb.broadcastStream(model.StreamEvent{Kind: model.StreamKindTextDelta, Delta: "Hel", Index: 0})
+	// seq, leaving the legacy stream_chunk shape out of the way. The loop stamps
+	// the session/turn identity on the event and the hub must carry it onto the
+	// frame, so a client can route the delta to the active session (design/16 §4.2).
+	hb.broadcastStream(model.StreamEvent{Kind: model.StreamKindTextDelta, Delta: "Hel", Index: 0, SessionID: "web", TurnID: "turn-abc"})
 	ev := readFakeFrame(t, c)
 	if ev.Type != "message.delta" {
 		t.Fatalf("frame type = %q, want message.delta (frame=%+v)", ev.Type, ev)
@@ -961,6 +963,9 @@ func TestHubBroadcastStream(t *testing.T) {
 	if ev.Seq != 1 {
 		t.Fatalf("frame seq = %d, want 1", ev.Seq)
 	}
+	if ev.SessionID != "web" || ev.TurnID != "turn-abc" {
+		t.Fatalf("frame scope = session %q turn %q, want session web turn turn-abc", ev.SessionID, ev.TurnID)
+	}
 
 	// A tool_call event maps to tool_call.*; a non-StreamEvent payload is dropped.
 	hb.broadcastStream(model.StreamEvent{Kind: model.StreamKindToolCallEnd, ID: "toolu_1", Index: 0})