feat(form-builder): per-schema default_crowd_type_id replaces silent oldest() heuristic (WS-6)

Session 2's PersonProvisioner picked CrowdType::oldest() for the org —
silently wrong for multi-crowd_type orgs (Volunteer + Crew + Press are
three distinct crowd_types in one org). Schemas now declare their
target crowd_type explicitly via form_schemas.default_crowd_type_id.
RequiresDefaultCrowdType publish guard prevents misconfigured
event_registration schemas from publishing.

PersonProvisioner: oldest() fallback removed entirely. Misconfiguration
throws no_default_crowd_type at runtime; publish guard prevents it at
config time.

Migration uses a plain ulid() column without DB-level FK because
SQLite's table-rebuild on ALTER ADD FOREIGN KEY cascade-deletes
form_fields rows (form_fields.form_schema_id has cascadeOnDelete on
form_schemas). Application-level integrity via FormSchema::defaultCrowdType()
belongsTo + the publish guard + the runtime failsafe — three load-bearing
checks, none of which require the DB-level constraint.

Three pre-existing migration backfill tests bumped step counts +1 to
account for the new migration sitting between WS-5c and WS-5d:
FormFieldBindingMigrationTest (16→17, 14→15), FormFieldConfigBackfillAndDropTest
(11→12), FormFieldValidationRuleBackfillTest (14→15),
ConditionalLogicBackfillTest (5→6).

Six event_registration test fixtures updated to set default_crowd_type_id
to satisfy the new publish guard.

FormBuilderDevSeeder.resolveDefaultCrowdTypeId() — VOLUNTEER → first-active
→ create-as-needed fallback chain; documented contract for future seeders.

SCHEMA.md updated to v2.7.
Refs: RFC-WS-6.md v1.1 §3 Q8 addendum (Task 4 of this session)

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

api/app/FormBuilder/Publishing/RequiresDefaultCrowdType.php

@@ -0,0 +1,33 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\FormBuilder\Publishing;
+
+use App\Models\FormBuilder\FormSchema;
+
+/**
+ * RFC-WS-6 v1.1 §3 Q8 addendum — event_registration schemas must
+ * declare a default_crowd_type_id so PersonProvisioner can land
+ * new registrants in the right CrowdType. Replaces the silent
+ * oldest() heuristic from session 2.
+ */
+final readonly class RequiresDefaultCrowdType implements PublishGuard
+{
+    public function code(): string
+    {
+        return 'requires_default_crowd_type';
+    }
+
+    public function evaluate(FormSchema $schema): PublishGuardResult
+    {
+        if ($schema->default_crowd_type_id !== null) {
+            return PublishGuardResult::passed($this->code());
+        }
+
+        return PublishGuardResult::failed(
+            $this->code(),
+            'form_builder_publish_guards.requires_default_crowd_type',
+        );
+    }
+}