Keep demos and figures visible in a side panel while the lesson continues

[RovleZMex]

Problem

In a teaching session the tutor built an interactive demo and kept referring to it for several turns ("move this in the demo and you'll see…"). Each time I had to scroll up to find the card, move the control, then scroll back down to read the reply and answer. The longer the lesson ran, the further away the demo was.

The teacher prompt asks for exactly this pattern (prompts/teacher/50-demos.md: "Teach WITH the artifact… Reference its controls and elements by name"), and it encourages several demos and figures per lesson, so I expect the scrolling to get worse in longer sessions.

Related work doesn't cover this case:

• feat: add fullscreen button to demo card #355 (demo fullscreen) covers the transcript, so I can't read the tutor's instructions or type while I use the demo.
• feat: per-lesson notes panel  #555 (notes panel) adds a right-side panel, but for the learner's own notes.

Proposal

A Demos panel on the right of the session view that keeps the session's demos and figures usable while the lesson continues.

• Opening it: an "Open beside" button on each demo and figure card, plus a top-bar chip (like Notes in feat: per-lesson notes panel  #555). It never opens on its own.
• One tab per artifact in the session, numbered in order. The active one fills the panel. When the tabs don't fit, they shrink to numbers.
• Go to message scrolls the transcript to where the artifact first appeared.
• No reload: while an artifact is in the panel, its card in the transcript shows a placeholder. The demo's iframe moves instead of remounting, so slider positions and running simulations survive the move (the same idea as feat: add fullscreen button to demo card #355).
• Layout: folds and resizes like the sidebar, and folds itself on narrow windows.
• Scope for a first version: demo and visual. show_image and mermaid charts could follow; mermaid lives inside message text, so it is a bigger change.

Questions

1. Does this fit in core, or would you rather it wait for the plugin infrastructure?
2. If feat: per-lesson notes panel  #555 lands, should demos and notes share one right-side panel (for example a Notes / Demos switch), or stay separate?
3. Should this build on feat: add fullscreen button to demo card #355's iframe-moving approach, or would you prefer a different way to keep demo state?
4. I left out auto-switching to the artifact the tutor mentions, because those mentions are free text. Is that the right call?

If the direction works for you, I'm happy to implement it.