Update SEQUENTIAL_QUEUE.md: persist-before-fire is now in effect

PR Expensify#91734 made push() async and await the Onyx disk commit before
flushing, closing the enqueue crash window the doc centred on. Rewrite
the affected sections to current reality:

- push() awaits persistencePromise before flush(false); enqueue write is
  now gated ahead of the network (was: fired off in-memory state)
- redraw the disk/network ordering diagram (commit precedes network)
- isReadyPromise mechanism: starts resolved, armed via
  setIsReadyPromisePending(); add isReadyPromisePending (7 state vars)
- four resolveIsReadyPromise sites (finally + all-empty/not-leader guards
  + offline-during-persist in push), not just the finally
- persist .catch handlers now Log.alert (storage emergency)
- note new SequentialQueueTest coverage; clarify persist-before-fire vs
  persist-before-optimistic (the latter still open)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: adhorodyski