submitTarget property
Null — every form outside a relation manager — submits to resource's
own endpoints exactly as before. Non-null redirects ONLY the write:
rendering, seeding, /state and /options all stay on the child
resource, because the child resource's form is what is on screen. This
is the whole write-target override (P9); nothing else about the form
knows it is editing a relation row.
Implementation
final RelationSubmitTarget? submitTarget;