docs(schema,arch-form-builder): reflect purpose registry v1.0

- SCHEMA.md §3.5.12 header rewritten for the 7-purpose vocabulary and
  `PurposeRegistry`. The `custom_purpose_slug` column is dropped from
  the `form_schemas` table and removed from the index list. The
  `form_submissions.subject_type` note cites
  `PurposeRegistry::allSubjectTypes()` instead of the deleted
  `config/form_subjects.php`.
- ARCH-FORM-BUILDER.md TL;DR updated: goal bullet cites 7 purposes
  (v1.0); §3.2 bullet notes the legacy 22-variant vocabulary is
  retired. §17.3 replaced: the "Custom purposes per organisation"
  section is gone; the new "Purpose registry" section documents the
  seven-slug table, PurposeDefinition shape, PurposeRegistry API,
  MorphMapAlignmentTest guard, the pre-publish binding check, and a
  step-by-step "adding a new purpose" checklist.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

dev-docs/SCHEMA.md

@@ -1790,11 +1790,14 @@ Immutable audit record of every email sent. No soft deletes.
 ## 3.5.12 Form Builder
 
 > Universal form builder — one schema/field/submission/value stack serving
-> 22 `FormPurpose` variants from `event_registration` to `incident_report`
-> to `signature_contract`. See `/dev-docs/ARCH-FORM-BUILDER.md` v1.2 for
-> the authoritative behaviour spec; this section documents the physical
-> tables as landed through S1 + S2a + S2b. Where ARCH §4 and the
-> migrations disagree, the migrations win (document code-as-built).
+> the seven v1.0 `FormPurpose` variants (`event_registration`,
+> `artist_advance`, `supplier_intake`, `post_event_evaluation`,
+> `incident_report`, `signature_contract`, `user_profile`) registered in
+> `config/form_builder/purposes.php` via `PurposeRegistry`. See
+> `/dev-docs/ARCH-FORM-BUILDER.md` v1.3 for the authoritative behaviour
+> spec; this section documents the physical tables as landed through
+> S1 + S2a + S2b + WS-2. Where ARCH §4 and the migrations disagree, the
+> migrations win (document code-as-built).
 >
 > **Legacy tables dropped (S2a):** `registration_form_fields`,
 > `person_field_values`, `registration_field_templates` removed by
@@ -1870,8 +1873,7 @@ that aggregates the user's submitted, non-test `form_submissions`.
 | `owner_id`                 | ULID nullable      | polymorph target                                                                               |
 | `name`                     | string             |                                                                                                |
 | `slug`                     | string             | canonical within organisation                                                                  |
-| `purpose`                  | string(50)         | `FormPurpose` enum value                                                                       |
-| `custom_purpose_slug`      | string nullable    | required when `purpose = custom`                                                               |
+| `purpose`                  | string(50)         | Slug matching a key in `PurposeRegistry::all()`; `FormPurpose` enum mirrors the seven v1.0 values |
 | `description`              | text nullable      |                                                                                                |
 | `is_published`             | bool               | default: false                                                                                 |
 | `submission_mode`          | string(20)         | `FormSubmissionMode` enum value                                                                |
@@ -1897,7 +1899,7 @@ that aggregates the user's submitted, non-test `form_submissions`.
 | `deleted_at`               | timestamp nullable | Soft delete                                                                                    |
 
 **Relations:** `belongsTo` organisation, createdBy/lastUpdatedBy/editLockUser (User); `morphsTo` owner; `hasMany` fields, sections, submissions, webhooks  
-**Indexes:** `(organisation_id, purpose)`, `(owner_type, owner_id)`, `(public_token)`, `(public_token_previous)`, `(custom_purpose_slug)`  
+**Indexes:** `(organisation_id, purpose)`, `(owner_type, owner_id)`, `(public_token)`, `(public_token_previous)`  
 **Unique constraint:** `UNIQUE(organisation_id, slug)`  
 **Global scope:** `OrganisationScope`  
 **Soft delete:** yes
@@ -2019,7 +2021,8 @@ that aggregates the user's submitted, non-test `form_submissions`.
 
 > One submission per `(schema, subject)` in `single` / `draft_single`
 > modes; unbounded in `multiple` mode. Polymorphic `subject_type` /
-> `subject_id` (allowed types per `config/form_subjects.php`).
+> `subject_id` (allowed types derived from
+> `PurposeRegistry::allSubjectTypes()`; WS-2 Q6 consolidation).
 > Carries the lifecycle timestamps, review status, optional schema
 > snapshot (when `form_schemas.snapshot_mode != 'never'`), locale used,
 > idempotency key, anonymisation marker, and a `search_index` text