Per-purpose schema validation composes a PurposeGuardProvider returning
a list of guards. Errors collected (not first-fail) so the builder UI
surfaces every issue per save. ConditionalRequirement composes higher-
order without proliferating one-off classes.
RequiresIdentityKeyBinding checks the is_identity_key flag specifically;
the binding-existence check is handled additively by the existing
assertRequiredBindingsPresent in FormSchemaService.
SchemaHasLinkedEvent checks owner_type='event' + owner_id (FormSchema
uses polymorphic owner; there is no direct event_id column).
i18n messages live in lang/nl/form_builder_publish_guards.php.
Refs: RFC-WS-6.md §3 (Q13), §4 (V1, V3)
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
