fix(form-builder): restore FK on form_schemas.default_crowd_type_id (WS-6)

The original session 2.5 migration had to omit this FK due to an
SQLite-only "rebuild on FK add" cascade-delete quirk. Now that the
test infrastructure has moved to MySQL (Task 1 of this session), the
quirk does not apply and the FK is restored to match every other FK
in this table.

Changes:
- New migration `2026_04_28_100000_restore_default_crowd_type_id_foreign_key`
  adds a FOREIGN KEY (default_crowd_type_id) REFERENCES crowd_types(id)
  ON DELETE SET NULL. Deleting a CrowdType nulls the column on dependent
  schemas instead of cascading the schema delete.
- Original migration's comment block rewritten — the SQLite-quirk
  rationale was demonstrably misleading; replaced with a forward-looking
  pointer to the FK-restore migration.
- PersonProvisioner::resolveCrowdTypeId() docblock updated: the runtime
  failsafe is now defense in depth alongside the DB-level FK + publish
  guard, not the sole load-bearing check.

New test (`DefaultCrowdTypeForeignKeyTest`) exercises both the
ON-DELETE-SET-NULL cascade and the existence of the FK in
information_schema.REFERENTIAL_CONSTRAINTS — the second assertion would
have been impossible on SQLite, which is exactly the point.

Migration step counts in 5 backfill tests bumped +1 because the FK-
restore migration sits at the top of the migration stack:
  - FormFieldBindingMigrationTest:           17→18, 15→16
  - ConditionalLogicBackfillTest:             6→7
  - FormFieldConfigBackfillAndDropTest:      12→13
  - FormFieldOptionsBackfillTest:             2→3
  - FormFieldValidationRuleBackfillTest:     15→16

All 1388 tests pass on MySQL (1386 prior + 2 new FK tests). Larastan
baseline unchanged.

Refs: RFC-WS-6.md v1.1 §3 Q9 addendum, WS-6 session 2.5 deviation #1

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

api/tests/Feature/FormBuilder/Bindings/FormFieldBindingMigrationTest.php

@@ -56,7 +56,7 @@ final class FormFieldBindingMigrationTest extends TestCase
         // validation-rules-backfill, create-validation-rules) +
         // 2 WS-6 migrations (action-failures, apply-status) +
         // 2 WS-5a migrations (drop-binding-cols, create-bindings) = 16.
-        $this->artisan('migrate:rollback', ['--step' => 17])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 18])->assertSuccessful();
         $this->assertFalse(Schema::hasTable('form_field_bindings'));
         $this->assertTrue(Schema::hasColumn('form_fields', 'binding'));
         $this->assertTrue(Schema::hasColumn('form_field_library', 'default_binding'));
@@ -118,7 +118,7 @@ final class FormFieldBindingMigrationTest extends TestCase
     public function test_rollback_reconstructs_json_and_drops_table(): void
     {
         // Walk back the full WS-5d + WS-5c + WS-6 + WS-5b + WS-5a stack (16 migrations).
-        $this->artisan('migrate:rollback', ['--step' => 17])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 18])->assertSuccessful();
         [$fieldAId] = $this->seedFieldsWithBindingJson();
         [$libAId] = $this->seedLibraryWithBindingJson();
 
@@ -133,7 +133,7 @@ final class FormFieldBindingMigrationTest extends TestCase
         // the pre-WS-5b state (conditional-logic, validation-rules, configs
         // and options tables gone, validation_rules + options JSON columns
         // reappear on source tables; binding contract intact).
-        $this->artisan('migrate:rollback', ['--step' => 15])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 16])->assertSuccessful();
         $this->assertFalse(Schema::hasTable('form_field_options'));
         $this->assertFalse(Schema::hasTable('form_field_conditional_logic_groups'));
         $this->assertFalse(Schema::hasTable('form_field_conditional_logic_conditions'));