Display `map` subepochs

[sneakers-the-rat]

So we have this unnamed notion of "map domains" which are basically anything that is downstream from a map node.

that should be displayed with colored area like how recursive tubes are, so that it's possible to know which things are running at a different cardinality.

map domains end at gathers and returns.

both of these features are possible to achieve without using the map or gather nodes, by design, so the visualization can't be perfect (unless we add more flags to nodes to allow them to declare that they have map or gather-like behavior