virtual/meta rooms

Nobody has claimed this yet.

note: this is a rough sketch

**Suggestion**

A room which doesn't have it's own timeline, but can merge + filter + sort events from other rooms. Maybe it could be impl'd as a normal room with a `m.room.virtual` state event? Would be useful to let the server do basic processing on large amounts events/data, instead of downloading everything then filtering locally.

examples:

- could be used for "matrix-matrix bridging" where one room aggregates events from multiple others
- aggregate threads from all rooms into one room
- could be used like `/notifications`

maybe something like this:

```js
{
"merge": {
"rooms": ["!foo"], // get events from these rooms
"spaces": ["!bar"], // get events from all rooms in these spaces
"types": ["m.something"], // get events from all rooms with this type
},

"filter": {
// maybe something like push rules, or implement a custom language for filtering
},

// maybe with bump_event_types, or again use a custom expr lang
"sort_by": ["recency"],
}
```

pretty low priority and probably extremely niche but is an interesting idea to put out there

The issue is a rough protocol sketch and names no files, tests, or implementation entry points. Begin by clarifying the proposed virtual-room model, merge and filtering semantics, and how completion would be validated in the Matrix specification.

Written by the indexing model from the issue text.