Make exclusions apply everywhere, including substitutions

Raised this as a bug and it turned out to be one, so this is the broader version. Exclusions should be a hard constraint applied at every point the app suggests an ingredient. Generation, substitution, scaling, the shopping list, all of it. Right now they are applied once at generation and everything downstream forgets. If I have excluded something it is usually because it makes the food inedible to me, not because I mildly prefer otherwise.