Harden the element input, fix its values and keep row identity on the server

Problem/Motivation

A crafted request can make the server build an unbounded number of rows, and an unexpected input shape ends in a 500 instead of a validation error. Several edge cases misbehave: unlimited cardinality renders no rows, #key on a single element is fatal, transforming children such as datelist never get their value, and the submit handlers rely on fixed parent offsets. Children with #access FALSE or #type value, and a disabled #key, travel as hidden inputs a user can change. Consumers also cannot decide when a row is empty or alter a single row.