Add histogram panel for masterclass invariant mass results

[rx18-eng]

summary:

Right now with the masterclass panel (#824), students can tag particles and get invariant mass values in a table. But the table
is just numbers , students can't really see the physics happening.

In tools like HYPATIA, students build up a histogram as they go through events. They tag particles, compute the mass, and it gets
added to a histogram. After 20-30 events you start seeing a peak around 91 GeV and that's when it clicks --- "oh that's the Z boson".
That moment doesn't happen with just a table of numbers.

The idea is to add a histogram panel (similar overlay style as the masterclass panel) that:

• Listens to invariant mass results as students compute them
• Fills a histogram bin each time
• Redraws live so students see the distribution building up
• Shows basic stats (entries, mean)
• Can export the data (CSV or image)

The cool part , jsroot (already a phoenix dependency, v7.9.1) has
createHistogram and draw/redraw APIs that we've never used. So this needs zero new dependencies.
This also connects to #826 (integration API) , the histogram panel would subscribe to the event bus (result-recorded events from the masterclass panel), which is a concrete proof that external tools can plug into phoenix through the bus.

Should be experiment-agnostic like the masterclass panel , ATLAS students histogram Z→ee/μμ masses, LHCb students could histogram D0→Kπ masses, same component different config.

This basically makes phoenix a complete replacement for standalone masterclass tools like HYPATIA --- the whole workflow (visualize
event → tag particles → compute mass → see histogram → discover
resonance) happens in one browser tab.

[rx18-eng]

@EdwardMoyse @sponce ,please let me know your thoughts on this !

[cohm]

FWIW, we organize ATLAS masterclasses in Stockholm on Monday, and I would be happy to test this on students then if it's available. I think the pedagogical advantage of being able to use a 3D tool where you can change perspective is huge, and really helps the students to get a better understanding.

Best,
Christian

[cohm]

In fact, I'll have a (very physics interested) 8th grader around for the next two weeks who could really help to test things like these!

[rx18-eng]

That's great to hear! I'll try to get this working as soon as possible. Would be really valuable to get feedback from actual students using it.

[rx18-eng]

hey @cohm, i've raised a PR for this (#843)you can check the implementation and the video i attached there. let me know if you'd like any changes or if something needs to work differently for your students!