feat(flow): 实时全链路流程可视化 P1——trace→graph_update→前端全链路流程页(design/19)

后端: trace/graph.go(RunGraph 图构建器,kind/status 映射,父子优先连线,脱敏白名单+截断);
       loop/flow.go(FlowPublisher 按会话增量发布 ws.graph_update,仅主人web会话); loop span
       start/finish 折入图; run.go StartRound/EndRound; wire.go 组装+注入.
前端: Flow.tsx(全链路流程页:FlowGraphView+会话/节点/开始/耗时卡片+状态计数+节点点开日志);
       FlowGraph 组件加 onNodeClick; types/wsParse 扩展; router/sections/AdminApp 注册 flow;
       api-client.md §3.5 graph_update 契约.
验证: 后端 build/vet/test 全绿+flow_test.go 集成(span→graph→ws.graph_update主干); 前端 build 绿.
红线: 单二进制零新依赖; 工具参数/结果脱敏; 仅主人web会话; 身份靠actor元数据; 只读不虚构.
P2(并行子代理分支/泳道/节点实时日志)后置.

internal/runtime/trace/span.go

@@ -158,6 +158,16 @@ func (s *Span) recordLocked() SpanRecord {
 	return rec
 }
 
+// Snapshot returns an immutable copy of a finished span as a SpanRecord. It is
+// only safe to call after Finish returns (a closed span is immutable), and it
+// lets a consumer read the span's final state without holding its lock. Calling
+// it on a non-finished span returns a record with a zero Status/EndedAt.
+func (s *Span) Snapshot() SpanRecord {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	return s.recordLocked()
+}
+
 // SpanRecord is an immutable, lock-free snapshot of a completed span, produced
 // by Finish and passed to a Writer for persistence. It is safe to copy.
 type SpanRecord struct {