Q6 of ARCH-CONSOLIDATION-ADDENDUM-2026-04-24: the allowed
`form_submissions.subject_type` values are now derived from
`PurposeRegistry::allSubjectTypes()` instead of the parallel
`config/form_subjects.php` file.
- CreateFormSubmissionRequest validates `subject_type` against the
registry via constructor-injected PurposeRegistry.
- FormSubmissionController and FormValueService resolve the subject
FQCN through `Relation::getMorphedModel()` — the morph-map is the
single source of truth for alias → model mapping.
- `config/form_subjects.php` is deleted. `MorphMapAlignmentTest` keeps
the registry and morph-map aligned going forward.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>