Need for support for purely observable self-updating state

## Describe the task

We continue running into the case of needing in state objects which are self-updating. Having these by the existing Feature paradigm means that they are effectively invalid on tick.

Examples:

- Player input snapshot; having one source of truth that can be accessed as state for current player input is highly useful, but as Feature would require dispatch/invalidation per frame
- Timers being held in state; countdowns and the like are useful to keep in state for cross-system synchronisation, but again would either require per-frame dispatch or per-frame invalidation of state

## Completed When

- [ ] A method of handling such cases is developed.

Start by reading the existing Feature paradigm and the state lifecycle around each tick. Compare the player input snapshot and timer examples, then define what a self-updating state object must do and how completion will be verified, since the issue currently names no files or tests.

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