Review on #2957: set_props took the field out of the tree before attach
checked the destination, so a refused move (a parent holding /MaxLen, say)
left it out of /Fields for a direct caller of the forms API. The read-only
check is now its own function, run before the field is detached.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>