9 backed string enums covering purpose, field type, submission status/mode/review,
field width, value storage hint, snapshot mode, webhook delivery status.
FormPurpose/FormFieldType include helper methods per ARCH §3/§5. All with
declare(strict_types=1) and values() helpers for validation rules.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>