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/session_ctx.go

@@ -19,3 +19,22 @@ func loopSession(ctx context.Context) string {
 	s, _ := ctx.Value(loopSessionIDKey{}).(string)
 	return s
 }
+
+// loopTurnIDKey is the context key for the assistant turn the current Run is
+// producing. The gateway attaches it (WithTurnID) before starting a turn so the
+// streaming events the loop emits can be correlated to that turn by the client.
+type loopTurnIDKey struct{}
+
+// WithTurnID attaches turnID to ctx so the streaming events the loop emits carry
+// the turn they belong to. It is exported because the gateway calls it once per
+// turn, before Run; a caller that attaches none leaves the events' TurnID empty,
+// which keeps the correlation fields backward compatible.
+func WithTurnID(ctx context.Context, turnID string) context.Context {
+	return context.WithValue(ctx, loopTurnIDKey{}, turnID)
+}
+
+// loopTurn returns the turn id on ctx, or "" when none was attached.
+func loopTurn(ctx context.Context) string {
+	s, _ := ctx.Value(loopTurnIDKey{}).(string)
+	return s
+}