Extract motion primitives from optimization

Related to #29: if we iteratively update delta, we can also extract valid motion primitives from the optimization. This will likely only have a minor impact on performance, though.

Start by reading related issue #29 and locating the optimization flow where delta is iteratively updated. The issue names no files or tests; done would require extracting valid motion primitives from that optimization while confirming the expected performance impact.

Written by the indexing model from the issue text.