feat: add masterclass panel and ATLAS masterclass landing page (close…

[rx18-eng]

Body

closes #835 and #915 in one PR. #835 was stale (branch drifted too far to rebase practically), so i rebuilt the masterclass code on a fresh branch from main instead.

what's in

• masterclass tagging panel. opt-in via uiConfig.showMasterclassPanel (default false so /atlas etc stay unchanged). tag particles, live invariant mass, results history.
• /atlas-masterclass route with a reduced bar. same atlas detector + event as /atlas. drops vr, ar, animate, clipping, overlay, share, make-picture. more info menu keeps info panel, event browser, eta-phi, kinematics (drops collections info + geometry browser).
• landing card between atlas and lhcb pointing to the new route.
• on() / emit() event bus on EventDisplay (addresses part of Allow easier integration of phoenix in other applications #826). standard events fired:
  • particle-tagged
  • particle-untagged
  • result-recorded

not in (intentionally)

• histogram. feat: add in-browser histogram panel using jsroot (#840) #843 stays separate, will rebase + follow up.
• LHCb / CMS masterclass configs. sebastien mentioned looping in bcouturi for LHCb so leaving that for later.
• docs (separate, addressing Add documentation about eta/phi heat map #911 Add documentation about Event Dataset browser #913 Add documentation about PHYSLITE ROOT file loader #914)

Recording.2026-06-02.002754.1.1.1.mp4

[rx18-eng]

@EdwardMoyse Let me know your thoughts on this !

[EdwardMoyse]

This looks great. I think we should get this in now, then I can share it with some masterclass folk. We can implement any feedback for 4.1.0