Enqueuing the sending of read receipts / unread markers while offline

Nobody has claimed this yet.

Consider the following scenario:

1. I have some unread messages in a room
2. I lose network connection
3. While offline, I open the room to read the messages I received previously
4. Any attempt to send read receipts or update unread markers fails due to lack of connectivity
5. I leave the room's timeline
6. Some time afterwards I regain network access
7. The room will continue to show as unread even though I have read it

It would be helpful if clients could enqueue the sending of read receipts to happen whenever the network is available. I'm interested in discussing if and how this might be possible.

Naively, I have considered pushing the sending of receipts through the send queue. Receipts are not room events, however, and I'm unsure if we'd really want them to mingle with the linearised handling of events?

Apart from this, two immediate complications I foresee:

1. We'd need a local echo for read receipts (see also https://github.com/matrix-org/matrix-rust-sdk/issues/4970)
2. Enqueuing a receipt can invalidate an enqueued earlier receipt. This is probably less involved than 1. and in a send queue implementation might fit into `canonicalize_dependent_requests`.

Start by examining the send queue and the canonicalize_dependent_requests entry point mentioned in the issue, then review the linked local-echo issue. The scope is still exploratory: done would require an agreed design for offline receipt queuing, local echo behavior, and invalidation of superseded receipts.

Written by the indexing model from the issue text.