Stop() called while a RestartAsync() is still pending is ignored, so the action keeps running after the caller stopped it #57

What's wrong

IntervalAction/IntervalAction.cs (around lines 149-179), RestartCoreAsync:

1. It awaits the previous restart (WaitAndDiscardOutcomeAsync(previousRestart)).
2. If polling was active, it calls Stop() and awaits the old PollingTask.
3. Then it runs lock (Lock) { ShouldPoll = true; … } unconditionally and starts a new polling loop.

If the caller calls Stop() while either await is pending, its ShouldPoll = false is simply overwritten in step 3. The instance ends up running even though the caller's most recent call was Stop(). The caller has no way to prevent this: Restart/RestartAsync take no cancellation token, and nothing records that a stop happened in the meantime.

Failure scenario

Setup: PollingInterval = 200 ms, ActionInterval = 0.

ia.Start();
await Task.Delay(50);
Task r = ia.RestartAsync();
ia.Stop();                 // caller wants it stopped
await r;
await Task.Delay(300);
// count action runs over the next second

• Actual: ShouldPoll == true, and the action ran 5 times during the second after Stop().
• Expected: 0 runs.

This happens in real use. A UI "Restart" click followed quickly by app shutdown, or a Dispose that calls Stop(), leaves a loop running against disposed state.

Related but different: #52 covers the opposite order (Stop() then Restart() leaving two loops alive). I applied the RestartCoreAsync change from open PR #55 and ran the same repro: it still fails with the same numbers. This needs its own fix.

Suggested fix / acceptance criteria

• Keep a stop/restart generation counter under Lock:
  • Stop() increments it.
  • RestartAsync captures the value it expects when it is called.
  • RestartCoreAsync sets ShouldPoll = true and starts the loop only if no Stop() has happened since.
• Alternatively, have Stop() cancel pending restarts through a CancellationTokenSource that RestartCoreAsync observes.
• Add a test: RestartAsync(); Stop(); await restart; leaves ShouldPoll == false and no action runs afterwards.