Add the layer's pose (authored as a delta) on top of the base.

Blend model-space rotations and local positions/scales using Esoterica's layer blending.

Apply model-space additive deltas using the base pose's original parent orientations.

Blend model-space rotations root-to-tip, preserving local positions and scale, as in Source 2.

Cross-fade the layer over the base by the layer weight.