Allocator need to access chunk headers which must be with a constant tag value (tag == 0) or elide tag checks on those checks. Current zimte spec uses pointer_tag == 0 as elision for tag checks. It is going to change as it provides bypass. If TG ends up defining a elide mechanism, that will be useful for these needs as well.
Focus on 4 bit tags but keep ISA (and thus allowing software as well) tagwidth agnostic.
