OptionaleventsOptionalsplitsSub-emitters triggered by parent particles. Each becomes its own
ParticlePass instance fed by a GPU spawn-request queue.
OptionalsdfBaked SDF bound to the particle pass. Required when the graph uses
sdf_attractor, sdf_collision, or an sdf_surface spawn shape; ignored
otherwise. Exactly one SDF per system (multiple SDF modifiers share it).
Full description of a particle system, consumed by the particle pass and shader builders.