Alternative threads UX: inline

### Your use case

#### What would you like to do?
i'm trying out labs threads, but they often don't feel great in the right side panel.
I imagine there has been a concept of inline UI for threads at some point.

compact, based on the unfolding reply threads feature of element:

compacter, more reaction-like

(these screenshots are narrow for demo purposes, but of course the messages would be sentence-length instead of single words)

#### Why would you like to do it?
I feel like switching to the panel
- is clunky & disconnected
- not so readable since the panel is ~20% wide, room list is 20% wide, meanwhile the usual timeline is the remaining 60% wide
- on top the whole panel UX is generally really uncomfortable and threads only make it worse

I get that one idea of using the panel is to be able to view both timelines at once. But in the way it is tone, this isn't useful in my eyes:
- I'd rather just reply normally to keep the flow of the main timeline
- switching between threads is tedious

#### How would you like to achieve it?
2 concepts above.
the idea is
- as long as the thread is short, thread replies are somewhere between an (emoji) reaction and a full blown comment. as such the UI design of the thread messages is extremely compact.
- threads stay in line, no disconnect from the normal timeline. multiple can be visible at once, e.g. if someone makes to points in a discussion, comments to each point can happen "in-line"
- long (> 5 replies?) threads can collapse this box and switch to panel mode. possibly keep showing the last couple replies and/or scrollbox similar to how code blocks now work
- preferably you can pop out a thread similar to the image lightbox.

### Have you considered any alternatives?

_No response_

### Additional context

_No response_

No repository files, tests, or entry points are named. Start by reviewing the existing labs threads experience and right-side panel against the two linked screenshots; done would require an agreed inline-thread design covering compact replies, long-thread collapse, and optional pop-out behavior.

Written by the indexing model from the issue text.