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/model/stream.go

@@ -57,4 +57,15 @@ type StreamEvent struct {
 	// ID is the provider block/index identifier for a tool call (e.g. Anthropic's
 	// tool_use block id). Empty for text segments.
 	ID string
+	// SessionID is the conversation the streamed event belongs to. It is stamped
+	// by the loop at the turn boundary (loop.streamStep), which reads the session
+	// the Run was started for; a model adapter, running below the loop and with no
+	// notion of a session, leaves it zero. A consumer that routes the event (the
+	// WS hub) reads it to target the right conversation.
+	SessionID string
+	// TurnID is the assistant turn the event belongs to. Like SessionID it is
+	// stamped by the loop, which reads the turn identity the caller attached with
+	// loop.WithTurnID; it is zero when no turn id was attached, so a consumer must
+	// treat both fields as best-effort correlation rather than a hard guarantee.
+	TurnID string
 }