feat(orchestration,gateway): 子代理结果登 WS 帧 subagent_result（裁决 02.4.3）

- 编排层改为在 subagent.result 上发布带父会话归属的 SubagentResultEvent，后台批次结果不再消失在进程内总线
- gateway 中继：新帧常量 wsSubagentResult="subagent_result"，eventBridge/bridgePatterns 登记该 topic，normalizeEvent 专用成形（帧壳 sessionId = 派生该批次的父会话）
- 角色门 fail-closed：未登记进 frameReachesAll，owner/admin 可达、viewer 不可达，真 JWT 三角色 WS 断言钉住
- 前景（2s 内跑完）批次仍只回工具结果，不发此帧

internal/runtime/orchestration/spawntool_test.go

@@ -176,10 +176,10 @@ func TestSpawnToolMovesSlowBatchToBackground(t *testing.T) {
 	s.runner = &mockRunner{gate: gate, result: &loop.Result{Text: "late", Steps: 2}}
 	sp := NewSpawnTool(s, WithSpawnWait(40*time.Millisecond))
 
-	results := make(chan *MergedResult, 1)
+	results := make(chan *SubagentResultEvent, 1)
 	unsub, err := s.Bus().Subscribe(context.Background(), SubagentResultSubject,
 		func(_ context.Context, e bus.Event) error {
-			if res, ok := e.Payload.(*MergedResult); ok {
+			if res, ok := e.Payload.(*SubagentResultEvent); ok {
 				select {
 				case results <- res:
 				default:
@@ -192,8 +192,11 @@ func TestSpawnToolMovesSlowBatchToBackground(t *testing.T) {
 	}
 	defer unsub()
 
+	// 派发回合的会话（loop 在 Run 顶部把它播进 ctx）：回填载荷必须带上它，
+	// gateway 的中继才能把结果归属到父会话的实时流（design/05 §9）。
+	ctx := loop.WithFlowBranch(context.Background(), parentSession)
 	start := time.Now()
-	raw, err := sp.Execute(context.Background(), map[string]any{"goal": "slow"})
+	raw, err := sp.Execute(ctx, map[string]any{"goal": "slow"})
 	elapsed := time.Since(start)
 	if err != nil {
 		t.Fatalf("Execute: %v", err)
@@ -212,14 +215,32 @@ func TestSpawnToolMovesSlowBatchToBackground(t *testing.T) {
 	close(gate)
 	select {
 	case res := <-results:
-		if res.Total != 1 || res.Succeeded != 1 {
-			t.Fatalf("background merge = %d total / %d ok, want 1/1", res.Total, res.Succeeded)
+		if res.SessionID != parentSession {
+			t.Fatalf("result sessionId = %q, want the dispatching session %q", res.SessionID, parentSession)
 		}
-		if res.Agents[0].Text != "late" {
-			t.Fatalf("background result text = %q, want late", res.Agents[0].Text)
+		if res.Status != "done" || len(res.Agents) != 1 {
+			t.Fatalf("result = %+v, want one settled agent", res)
+		}
+		if a := res.Agents[0]; a.Text != "late" || a.Steps != 2 || a.Goal != "slow" || a.Status != string(StatusDone) {
+			t.Fatalf("result agent = %+v, want the late/2-steps/done projection", a)
+		}
+		// 载荷形状是 api-client §3.10 登记的 WS 帧 data：键集锁定，避免
+		// 生产者悄悄改键而文档与实现分叉。
+		wire, merr := json.Marshal(res)
+		if merr != nil {
+			t.Fatalf("marshal result payload: %v", merr)
+		}
+		var keys map[string]json.RawMessage
+		if uerr := json.Unmarshal(wire, &keys); uerr != nil {
+			t.Fatalf("unmarshal result payload: %v", uerr)
+		}
+		for _, want := range []string{"sessionId", "status", "summary", "agents"} {
+			if _, ok := keys[want]; !ok {
+				t.Fatalf("result payload %s lacks key %q", wire, want)
+			}
 		}
 	case <-time.After(waitTimeout):
-		t.Fatal("a background batch never published its merged result")
+		t.Fatal("a background batch never published its result payload")
 	}
 }