定位有效性生命周期

stateDiagram-v2
  [*] --> NoFix
  NoFix --> Candidate: sample
  Candidate --> Valid: accepted
  Candidate --> NoFix: rejected
  Valid --> Stale: freshness deadline
  Stale --> Valid: fresh sample
  Valid --> NoFix: source lost

有坐标不等于 Valid；Stale 不能继续伪装为实时位置。