Aggregation API v2: Spec holes for non live timeline

For non live timelines (or gappy syncs), find a way to:
- [ ] know that I have reacted
- [ ] unreact one of my reaction (related to the above one)
- [ ] handle unreact reception

This may be hard to do without spec update.

Start by reviewing the Aggregation API v2 behavior for non-live timelines and gappy syncs, then trace how reactions are represented and handled in this repository. The work is done when the three cases—knowing about your reaction, unreacting it, and receiving an unreact—have an agreed specification and implementation path.

Written by the indexing model from the issue text.