Adds a multi-burst particle firework system to the Nexus 3D scene.
Each firework is a spherical explosion of colored particles with
additive blending and physics-based trajectories (gravity applied
analytically from burst origin).
Triggers:
- `milestone-complete` custom window event (for infra integration)
- chat messages containing "milestone" keyword
- 6 staggered bursts, random positions above the platform, 7-color palette
Fixes#254
