feat: add event dataset browser with physics-aware filtering

[rx18-eng]

Problem

When loading multi-event files (PHYSLITE, CMS .ig, EDM4hep), Phoenix shows a simple dropdown with event keys ("Event 1", "Event 2"...). There's no way to know what's inside each event without loading it. Users click through events blindly looking for interesting topologies.

For PHYSLITE files with 100+ events, or masterclass datasets with 50 events, this wastes significant time. Students spend more time clicking "next" than analyzing physics.

Proposed Solution

An event dataset browser panel that pre-scans all loaded events and shows a sortable, filterable physics summary.

Core features:

• Pre-scan events on load, build summary index (object counts per collection, MET, total energy)
• Sortable table (click column header to sort by # jets, # muons, MET, etc.)
• Filter controls (e.g. "events with >= 2 muons", "MET > 20 GeV")
• Click a row to load that event
• Keyboard navigation (arrow keys to browse, Enter to load)
• Active event highlighted in the table

Design principles:

• Config-driven columns and filters (experiment-agnostic, works for ATLAS/CMS/LHCb/FCC)
• No new dependencies (reuses existing Angular patterns)
• Works with any data format that Phoenix supports

Example column config:

{
  columns: [
    { key: 'eventNumber', label: 'Event' },
    { key: 'Jets', label: 'Jets', countCollection: true },
    { key: 'Tracks', label: 'Tracks', countCollection: true },
    { key: 'Muons', label: 'Muons', countCollection: true },
  ],
  filters: [
    { field: 'Muons', operator: '>=', label: 'Min Muons' },
    { field: 'MET', operator: '>=', label: 'Min MET (GeV)' },
  ]
}

Use Cases

• Students in masterclasses filtering to "2 muon events" to find Z candidates instead of clicking blindly through 50 events
• Researchers finding events with specific topologies (high MET, many jets, isolated leptons)
• Developers verifying that their loader produces the expected object counts across events
• Presenters picking the cleanest event for a talk or demo

Additional Context

Currently the event selector is a <select> dropdown (event-selector.component.ts) with no search, filter, or preview capability. Events are already all held in memory (eventsData in EventDisplay), so pre-scanning requires no additional data loading.

No keyboard shortcuts exist for event navigation (next/prev). This feature would also add arrow key and Enter support for event browsing.

[rx18-eng]

@sponce @EdwardMoyse , please let me know what you think of this !

[sponce]

Very interesting feature I must say. One thing that comes to my mind is the memory footprint of this. Events tend to be quite big and loading many of them (50, 100) may be a problem if one does not cleanup their data from memory. Also it may be relatively slow to scan all events.
But again, can be super nice to have

[EdwardMoyse]

I think this is a very nice idea - I guess you do not need to have the events in memory once the summary is built?

Very often people know that it is Event#123456 in a file which is the one they want to make a nice picture of, so even just being able to open a file, open a browser, and jump directly to the event in question would be fantastic. But I love the idea of filters - I'm not aware of other event displays with this functionality!

[rx18-eng]

Thanks for the feedback @sponce @EdwardMoyse! The scan is lightweight (just counting collection keys in JSON), so it's fast even for 100 events. The memory concern is valid - a future optimization could scan once, build summaries, then free raw data
and lazy-load individual events on demand. The browser UI would work the same either way. I'll also add a search box to jump
directly to a specific event number. Have a working implementation already, will open a PR soon.