#3598977 Other/composite/multiple elements & submission

Expose the *_other element family (select/radios/checkboxes other) and their other-option label + synthesized "other" sub-input, and accept "other" free-text values on submission, including when nested inside a composite element. Resolve element descriptions regardless of #multiple, and resolve multipleValues correctly for hardcoded-multiple plugins (checkboxes / *_other / custom composite).

Closes #3598977