Function shouldWriteLiveActual

Decide whether to write a new actual value onto a leg.

• null extracted → never write (transient ESPN miss / partial response; writing null over a real value would clobber good data).
• extracted equals current → skip the write entirely (no MVCC churn, no liveSnapshot.lastLiveStatAt drift).
• extracted is a finite number that differs from current → write.

Caller treats false as a no-op for that leg.