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/runtime/loop/stream.go

@@ -32,6 +32,12 @@ func (l *Loop) streamStep(ctx context.Context, messages []model.Message) (*stepO
 	}
 
 	resp, err := adapter.ChatStream(ctx, req, func(ev model.StreamEvent) {
+		// Stamp the session/turn identity at the loop boundary: the adapter runs
+		// below the loop and cannot know which conversation the stream belongs to,
+		// so the loop fills it from the context it was driven with (design/16 §4.2)
+		// and the frontend can route the delta to the active session.
+		ev.SessionID = loopSession(ctx)
+		ev.TurnID = loopTurn(ctx)
 		// Suppress UI broadcast for internal autonomous passes (backthink),
 		// whose intermediate reasoning must not leak into the chat stream.
 		if l.onStream != nil && !isMutedBroadcast(ctx) {