ichabod
Legned
You're saying there needs to be an exception to a rule, but you can't state the rule that you need an exception to. And I think if you take the rules as a whole, the combat turn is obviously meant to be fluid. You can break up your move, you can break up your action with your move, you can do your bonus action whenever you want to, you can break up actions with reactions, you can do a free object interaction whenever you want to. Nowhere does it state what order things have to happen on your turn. That fluidity is what you need an exception to. And there isn't one.The principles of exception based rule design coupled with the rules as a whole all taken together.
There is no single line, but there doesn’t need to be (see the example of humans and fire resistance above). If this was allowed the rules would say so somewhere - even if it’s just an indirect, you can take an action anytime on your turn.
Humans and fire resistance isn't a good example. There are rules that your features come from your race and your class, and the race has to explicitly grant them (PHB p. 11).