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

web/hwlab-cloud-web/src/api/workbench.ts

@@ -191,6 +191,9 @@ function normalizeTimingProjection(value: unknown): WorkbenchTurnTimingProjectio
   const lastEventAt = textValue(source.lastEventAt);
   const finishedAt = textValue(source.finishedAt);
   const durationMs = numberValue(source.durationMs);
+  // observedAt and lastEventAgeMs are server-side diagnostic snapshots.
+  // They must NOT be used as the user-visible relative time authority.
+  // Running relative time display must use startedAt/lastEventAt + browser local now().
   const observedAt = textValue(source.observedAt);
   const lastEventAgeMs = numberValue(source.lastEventAgeMs);
   if (!startedAt && !lastEventAt && !finishedAt && durationMs == null && lastEventAgeMs == null) return undefined;