feat: animate pileup interactions sequenced by timestamp (#235)

[deveshbervar]

Closes #235

What this adds

Animates multiple pp interactions (pileup) from a single bunch crossing,
sequenced in time order using the timestamp stored on each interaction.
This is the foundational feature needed for HL-LHC precision-timing
detector visualisation in ATLAS and CMS.

Demo

Recording.2026-03-28.143749.mp4

How it works

• Added PileupInteraction type with timestamp (ps), vertex (x,y,z), tracks and jets
• Added PileupEvent type containing an array of interactions
• animatePileup() sorts interactions by timestamp and maps each to a
  proportional delay, firing a vertex flash at the exact (x,y,z) position
• _animateInteractionTracks() handles per-interaction track animation
  when real tagged track data is provided
• New "Animate pileup" button added to the toolbar

Testing

• All 179 existing tests passing, 0 failures
• Tested on ATLAS sample: 10 sequential vertex flashes visible along
  beam axis, each clearly separated by ~1 second

Next steps for real HL-LHC data

Replace synthetic test data in animate-event.component.ts with real
JSON matching the PileupEvent format:

{
  "interactions": [
    { "timestamp": 0,  "vertex": {"x":0,"y":0,"z":-45}, "tracks":[] },
    { "timestamp": 32, "vertex": {"x":0,"y":0,"z": 12}, "tracks":[] }
  ]
}

[EdwardMoyse]

Sorry for taking so long to get back to you on this - I really wasn't quite sure what to say. Unfortunately I really think we should just close this PR - from the animation I can see that this is just fundamentally incorrect ... the pileup should be overlapping not played one after another. This is just not what is happening in reality. But I don't have time right now to try to work out if there is a correct way to do this - I suspect the timing differences from the different pileup will be imperceptible anyway.