Spawns particles on the surface of the bound ParticleGraphConfig.sdf.
Picks a random point inside the SDF's AABB and projects it to the nearest
surface along the gradient; initial velocity is the outward normal scaled
by initialSpeed. thickness ≥ 0 jitters the projected position along
the normal so the spawn band has finite width (default 0).
Geometric region used by an emitter to choose initial particle position and velocity direction.