feat(form-builder): FormFieldConditionalLogicService + cycle detection + legacy backfill + snapshot

WS-5c commit 2 of 4 — the service layer, backfill migration, and
read-path switch. Per addendum Q3, conditional_logic applies to
FormField only — no library mirror and no copyLogic on
FormFieldService::insertFromLibrary.

FormFieldConditionalLogicService owns every write:
  - logicFor(field): depth-limited eager-load of the tree
  - replaceLogic(field, tree): transactional structure + operator +
    field_slug validation + cycle check + activity-log emit
    (field.conditional_logic_replaced)
  - toJsonShape(root): reconstructs the canonical ARCH §8
    `{show_when: {...}}` shape — single source of truth for the
    snapshot writer + API resources
  - assertSpecsValid(tree): public boundary guard for the FormRequest
    strict validator (WS-5c commit 3 wires this up)
  - assertNoCycles(field, tree): contract preserved from
    FormFieldService::assertNoConditionalCycle, implementation now
    reads the relational adjacency.

Backfill migration translates pre-WS-5c conditional_logic JSON to
rows. Strict dispatch: unknown operators / unknown top-level keys /
malformed groups FAIL the migration — Phase A seed-scan confirmed
the catalogue parity, so any drift is a data bug to fix at source,
not silently absorb. Rollback rebuilds canonical JSON and clears
the relational tree.

FormFieldService.create/update route `conditional_logic` through
the new service (matching the extract-and-delegate pattern from
WS-5a bindings and WS-5b validation rules). Snapshot writer + both
resources (FormFieldResource, PublicFormSchemaResource) read via
`toJsonShape(rootConditionalLogicGroup())` — byte-for-byte parity
with the pre-WS-5c JSON contract.

InvalidConditionalLogicSpecException handled in FormFieldController
as 422, same as FrozenSchemaException / CyclicDependencyException.

Tests: 20 new under tests/Feature/FormBuilder/ConditionalLogic/
(service, cycle detection, backfill forward+rollback+failure cases,
snapshot + resource parity). FormFieldApiTest cyclic rejection test
rewritten to use the new factory state. Rollback step counts in
WS-5a/b migration tests bumped +1 for the new backfill migration.
Baseline 1122 → 1142 green (3032 → 3085 assertions).

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

api/app/Http/Resources/FormBuilder/FormFieldResource.php

@@ -8,6 +8,7 @@ use App\Enums\FormBuilder\FormFieldType;
 use App\Models\FormBuilder\FormField;
 use App\Models\PersonTag;
 use App\Services\FormBuilder\FormFieldBindingService;
+use App\Services\FormBuilder\FormFieldConditionalLogicService;
 use App\Services\FormBuilder\FormFieldConfigService;
 use App\Services\FormBuilder\FormFieldValidationRuleService;
 use App\Services\FormBuilder\FormLocaleResolver;
@@ -60,7 +61,9 @@ final class FormFieldResource extends JsonResource
             'binding' => app(FormFieldBindingService::class)->toJsonShape(
                 $this->resource->bindings->first(),
             ),
-            'conditional_logic' => $this->conditional_logic,
+            'conditional_logic' => app(FormFieldConditionalLogicService::class)->toJsonShape(
+                $this->resource->rootConditionalLogicGroup(),
+            ),
             'role_restrictions' => $this->role_restrictions,
             'translations' => $this->translations,
             'value_storage_hint' => $this->value_storage_hint instanceof \BackedEnum ? $this->value_storage_hint->value : $this->value_storage_hint,

api/app/Http/Resources/FormBuilder/PublicFormSchemaResource.php

@@ -9,6 +9,7 @@ use App\Models\FormBuilder\FormField;
 use App\Models\FormBuilder\FormSchema;
 use App\Models\PersonTag;
 use App\Models\Scopes\OrganisationScope;
+use App\Services\FormBuilder\FormFieldConditionalLogicService;
 use App\Services\FormBuilder\FormFieldConfigService;
 use App\Services\FormBuilder\FormFieldValidationRuleService;
 use Illuminate\Http\Request;
@@ -89,7 +90,9 @@ final class PublicFormSchemaResource extends JsonResource
                     'configs' => app(FormFieldConfigService::class)->toJsonShape($f->configs),
                     'is_required' => (bool) $f->is_required,
                     'display_width' => $f->display_width instanceof \BackedEnum ? $f->display_width->value : $f->display_width,
-                    'conditional_logic' => $f->conditional_logic,
+                    'conditional_logic' => app(FormFieldConditionalLogicService::class)->toJsonShape(
+                        $f->rootConditionalLogicGroup(),
+                    ),
                     'sort_order' => (int) $f->sort_order,
                     'form_schema_section_id' => $f->form_schema_section_id,
                 ];