form.Container: adjustTreeLeaves() => sharper separation of the key & value realms

when using a deeply nested form field names structure including objects & arrays, we need a smart way to check when we are leaving field paths and getting into the value realm.

the current version will break for checkbox groups, containing an array of items.

we should check for objects & arrays if a path matches a form field (then entering the value realm) or if it is not (staying inside the key realm).

i will dive into this tomorrow.