feat: PJ2026-0104010803 P4 前端/CLI 单读源收敛 - 删除 events.at(-1) 读侧推理 (#1758)

workbench.ts (stores):
- applyTraceHydrationResult: activityLabel 只读 result.lastEventLabel，不读 events.at(-1)
- lastEventLabel: 只读 result.lastEventLabel，不 fallback 到 events.at(-1)
- normalizeTraceSnapshot: lastEventLabel 只读 source.lastEventLabel
- mergeRunnerTrace: lastEventLabel 不从 events.at(-1) 推断

useTraceSubscription.ts:
- lastEventLabel 只读 snapshot.lastEventLabel，不读 events.at(-1)

trace-renderer.ts (CLI):
- traceNoiseSummaryRow: status 只读 trace.status/traceStatus，不 fallback 到 lastEvent.status
- 保持 lastLabel 用于展示，但不参与 lifecycle 推断

api/workbench.ts:
- 添加注释标记 observedAt/lastEventAgeMs 为 diagnostic-only，不作用户可见相对时间权威

Closes #1750
Refs #1742

tools/src/hwlab-cli/trace-renderer.ts

@@ -220,7 +220,7 @@ function traceCompletionSummaryRow(trace: Record<string, unknown>, event: TraceE
 
 function traceNoiseSummaryRow(trace: Record<string, unknown>, events: TraceEvent[], options: TraceDisplayRowsOptions): TraceEventRow {
   const lastEvent = events.at(-1);
-  const status = nonEmptyString(trace.status ?? trace.traceStatus ?? lastEvent?.status) ?? "running";
+  const status = nonEmptyString(trace.status ?? trace.traceStatus) ?? "running";
   const lastLabel = lastEvent ? readableTraceLabel(lastEvent) : "未观测";
   return {
     rowId: "trace-noise-summary",