Create more intentional separation between structural & progress data

[BenHenning]

It's not currently clear in the domain layer when a controller should produce read-only data (e.g. #121), produce read-only structural data with progress (#118), or specifically focus on progress (#119). It's likely that these are muddled because there aren't clear separations between:

1. Controllers that provide structural data (e.g. exploration structure)
2. Controllers that own persistent progress (e.g. for stories)
3. Controllers that own ephemeral progress (e.g. for explorations)
4. Controllers that provide UI-specific data (most controllers being introduced)

These ought to be clearly separate (probably via naming). It may also be nice if UI controllers are the primary controllers interacted with the UIs, and those depend on structural/progress controllers for sourcing data. 'Repository' may actually be a better name for these lower-level controllers.

Note also that structural controllers/repositories also need to have referential integrity, which likely requires #12 to be utilized for on-disk storage.