Making every pinned field read-only left an optional one unreachable: the
schema is equally happy with or without it, but the form displayed a value the
user could neither send nor edit. An optional `const` is a yes/no rather than
a fixed answer, so it renders as its single choice, clearable — opting in sends
the schema's own typed value, opting out leaves the property absent. A required
one is still displayed read-only, since there is nothing to decide.
Signed-off-by: cliffhall <cliff@futurescale.com>
e0ecf58df3