An action that throws after Stop() is called is silently lost: RethrowExceptions() reports nothing #56

What's wrong

RethrowExceptions() (IntervalAction/IntervalAction.cs:260-266) inspects only PollingTask.Exception. An action's exception reaches PollingTask only when a later polling tick calls TryRun(), which sees the faulted ActionTask and throws (:207-215).

If Stop() is called while an action is still running, the polling loop exits normally and no later tick happens. When that in-flight action then throws, its exception stays on the faulted ActionTask, where nothing ever reads it. A later Restart() then discards it on purpose (:170-177, _ = ActionTask.Exception; ActionTask = null;).

Repro (verified against the built library)

var ia = IntervalAction.Start(new IntervalActionOptions
{
    PollingInterval = TimeSpan.FromMilliseconds(50),
    ActionInterval = TimeSpan.FromHours(1),
    Action = () => { Thread.Sleep(300); throw new InvalidOperationException("boom"); },
});
await Task.Delay(100);
ia.Stop();                // action still running
await Task.Delay(1000);   // action has thrown by now
ia.RethrowExceptions();   // Actual: returns normally. Expected: throws "boom".

Without Stop(), the same setup throws "boom" from RethrowExceptions() as documented.

Why it matters

The README and CLAUDE.md say that exceptions from actions are captured and can be rethrown with RethrowExceptions(). The natural shutdown sequence, Stop() then RethrowExceptions(), swallows a failure from the final run without any sign, which is exactly when a caller checks for errors.

Suggested fix

• In RethrowExceptions(), when PollingTask has not faulted, also check ActionTask is { IsFaulted: true } and throw ActionTask.Exception.GetBaseException(). Alternatively, have the loop await and observe the in-flight ActionTask before it exits after a stop.
• Decide explicitly whether Restart() should keep an unobserved action exception instead of discarding it.

Acceptance: a test that throws from an action still running when Stop() is called sees that exception from RethrowExceptions() once the action has finished.