bug(timeline): Read receipts for a user can appear before they've joined a room

## STR

- create a timeline that tracks read receipts on message-like events
- send a message "hello world" in a room
- invite user A in the room
- user A joins the room

## expected

- user A's read receipt should be on their join state event, or there should be none

## observed

- user A's read receipt is on the "hello world" message that preceded their joining

Reproduce the timeline scenario with a prior message, an invitation, and the user's join event, then inspect the read-receipt handling and timeline entry points. Done means the user's receipt is attached to their join state event or is absent, rather than appearing on the earlier message.

Written by the indexing model from the issue text.