formSchema now filters type: widget fields by the action's permissions,
the same D-12 filtering listSchema applies to toolbarActions, so an admin
without the action permission no longer gets a button that always
answers 403, and the action name is not revealed. The filtered fields are
a new slice, so the cached schema is never modified.
isJSONScalar now encodes each value and keeps it only when the encoding
is a string, number, boolean or null, so a named scalar whose MarshalJSON
writes an array or object, NaN and the infinities are dropped. writeJSON
encodes into a buffer before the status line, so an encode failure is a
logged 500 with the generic envelope instead of a 200 with a truncated
body.
