Rename .meta({ casRef: true }) to .meta({ 'x-cas-ref': true }) across
all schema annotations. Zod v4 toJSONSchema automatically outputs
x-cas-ref in the generated JSON Schema, so buildDescriptor preserves
the annotation without any code changes.
Add buildDescriptor test verifying x-cas-ref appears in descriptor
output for annotated fields and is absent for plain fields.
220 tests pass, 0 fail.
Fixes#291, Refs #285
xiaoju
4e126a258a