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/tests/Feature/FormBuilder/ValidationRules/FormFieldValidationRuleBackfillTest.php

@@ -37,7 +37,7 @@ final class FormFieldValidationRuleBackfillTest extends TestCase
         // validation-rules-backfill + create-validation-rules) = 7.
         // Brings us to the pre-WS-5b state: validation_rules JSON column
         // present, no relational tables for WS-5b.
-        $this->artisan('migrate:rollback', ['--step' => 7])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 8])->assertSuccessful();
         $this->assertFalse(Schema::hasTable('form_field_validation_rules'));
         $this->assertTrue(Schema::hasColumn('form_fields', 'validation_rules'));
 
@@ -98,7 +98,7 @@ final class FormFieldValidationRuleBackfillTest extends TestCase
         // (validation_rules JSON column present; no relational tables for
         // WS-5b). Step count: drop-cols + configs-backfill + create-configs
         // + validation-rules-backfill + create-validation-rules = 5.
-        $this->artisan('migrate:rollback', ['--step' => 7])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 8])->assertSuccessful();
 
         $fieldId = $this->seedFieldWithJson([
             'field_type' => 'TAG_PICKER',
@@ -122,7 +122,7 @@ final class FormFieldValidationRuleBackfillTest extends TestCase
         // (validation_rules JSON column present; no relational tables for
         // WS-5b). Step count: drop-cols + configs-backfill + create-configs
         // + validation-rules-backfill + create-validation-rules = 5.
-        $this->artisan('migrate:rollback', ['--step' => 7])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 8])->assertSuccessful();
 
         $fieldId = $this->seedFieldWithJson([
             'field_type' => 'TEXT',
@@ -149,7 +149,7 @@ final class FormFieldValidationRuleBackfillTest extends TestCase
         // (validation_rules JSON column present; no relational tables for
         // WS-5b). Step count: drop-cols + configs-backfill + create-configs
         // + validation-rules-backfill + create-validation-rules = 5.
-        $this->artisan('migrate:rollback', ['--step' => 7])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 8])->assertSuccessful();
 
         $this->seedFieldWithJson([
             'field_type' => 'TEXT',
@@ -166,7 +166,7 @@ final class FormFieldValidationRuleBackfillTest extends TestCase
         // (validation_rules JSON column present; no relational tables for
         // WS-5b). Step count: drop-cols + configs-backfill + create-configs
         // + validation-rules-backfill + create-validation-rules = 5.
-        $this->artisan('migrate:rollback', ['--step' => 7])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 8])->assertSuccessful();
 
         $this->seedFieldWithJson([
             'field_type' => 'BOOLEAN',
@@ -185,7 +185,7 @@ final class FormFieldValidationRuleBackfillTest extends TestCase
         // full-back-then-full-forward cycle — rolling back all WS-5b
         // migrations restores the pre-WS-5b state (columns present on
         // source tables; validation rules relational table gone).
-        $this->artisan('migrate:rollback', ['--step' => 7])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 8])->assertSuccessful();
         [$numberId] = $this->seedFields();
 
         $this->artisan('migrate')->assertSuccessful();
@@ -200,7 +200,7 @@ final class FormFieldValidationRuleBackfillTest extends TestCase
 
         // Roll back WS-5b fully → column reappears and carries canonical JSON
         // reconstructed from the relational rows.
-        $this->artisan('migrate:rollback', ['--step' => 7])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 8])->assertSuccessful();
         $this->assertTrue(Schema::hasColumn('form_fields', 'validation_rules'));
 
         $field = DB::table('form_fields')->where('id', $numberId)->first();