splat_velocity_merge

Merges the Gaussian-splat velocity accumulation into the frame's velocity buffer, so every consumer of that buffer — motion blur, temporal antialiasing, temporal upsampling, a denoiser's history term — sees a moving cloud move.

What it reads

binding	holds
splat_velocity	xy = each splat's NDC displacement summed premultiplied by its alpha, w = the coverage those splats add up to. The splat pass writes it as the second target of the Gaussian channel, composited "over" in the same sorted order as the colour.
scene_motion	The frame's velocity buffer as the geometry passes left it.
merged_out	Where the merged field lands. A copy lays it back over @scene.motion, because a pass cannot sample the attachment it renders into.

The weight

w is coverage, and coverage is the weight. A texel the cloud fully covers reports the cloud's displacement; a texel it half covers reports halfway between the cloud and the surface behind it; a texel it never reached keeps what the geometry passes wrote, bit for bit. That is the same weight the cloud's colour arrived at the texel with, so what a velocity consumer treats as moving is what the image shows moving.