fix(form-builder): canonicalize JSON for byte-stable storage (WS-6)

MySQL 8.0 JSON columns may reorder associative-array keys on
round-trip. For audit-immutable values (schema snapshots, webhook
payloads, activity log diffs), this is corrupting: re-emits produce
different byte sequences for the same logical content.

Introduced JsonCanonicalizer (recursive ksort on associative arrays;
numeric-indexed lists preserve order) and applied at every writer
site that produces byte-stable JSON:

- FormSubmissionService: canonicalize the schema_snapshot array
  before storage (audit-immutable per ARCH §4.3, RFC-WS-6 v1.1).
- FormField::logFieldChange / FormSchema::logSchemaChange: canonicalize
  activity-log properties before withProperties() so old/new diffs
  read back byte-stable.
- BindingActivityLogger: canonicalize both the pass-level and
  per-binding activity properties.
- FormWebhookDispatcher: canonicalize payload_snapshot before
  storage (delivery-time HMAC re-encodes the same canonical bytes).
- DeliverFormWebhookJob: switched json_encode to
  JsonCanonicalizer::encode for the HMAC-signed body, so the
  signature is byte-stable across re-deliveries and reproducible by
  receivers from the same logical payload.

Sites NOT canonicalized (deliberate):
- form_schemas.settings — opaque UI config; key order has no
  semantic meaning, no byte-stability requirement.
- form_schemas.translations / form_fields.translations — read by
  display layer; key order doesn't matter.
- form_templates.schema_snapshot — user-supplied input via store/
  update; user is the source of truth, not audit-immutable in the
  same way as form_submissions.schema_snapshot.

Reverted the 7 assertEquals workarounds from session 2.6:
- ConditionalLogicActivityLogPayloadTest
- ConditionalLogicBackfillTest::test_rollback_reconstructs_canonical_json
- FormFieldBindingMigrationTest::test_rollback_reconstructs_json_and_drops_table
- FormFieldOptionServiceAndScopeTest::test_replace_options_emits_activity_log_on_field_only
- FormFieldOptionsActivityLogTest::test_field_updated_payload_contains_options_diff_when_options_change
- FormFieldOptionsBackfillTest::test_forward_migration_backfills_rows_strips_translations_and_rewrites_snapshot
- FormFieldOptionsSnapshotAndStrictRequestTest::test_submission_snapshot_embeds_rich_shape_options

Each now uses assertSame on JsonCanonicalizer::encode of both sides —
byte-stable comparison meaningful regardless of MySQL JSON storage
behavior.

New regression test SchemaSnapshotByteStableAcrossReemitsTest
exercises the contract end-to-end: complex schema with bindings,
validation rules, options, conditional logic, submitted; reads
schema_snapshot via three roads (Eloquent cast, fresh model, raw
bytes) and asserts the canonical encode is identical.

ARCH-FORM-BUILDER.md §4.6.1 gets a "Byte-stability" sub-section
explaining what's canonicalized and why.

Test count: 1388 → 1400 (+11 JsonCanonicalizer unit, +1 snapshot
regression). Larastan clean. Rector dry-run unchanged at 355.

Refs: WS-6 session 2.6 deviation #4 cleanup, RFC-WS-6 v1.1

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

api/tests/Feature/FormBuilder/Options/FormFieldOptionsActivityLogTest.php

@@ -11,6 +11,7 @@ use App\Models\FormBuilder\FormSchema;
 use App\Models\Organisation;
 use App\Services\FormBuilder\FormFieldOptionService;
 use App\Services\FormBuilder\FormFieldService;
+use App\Support\Json\JsonCanonicalizer;
 use Illuminate\Foundation\Testing\RefreshDatabase;
 use Spatie\Activitylog\Models\Activity;
 use Tests\TestCase;
@@ -61,22 +62,23 @@ final class FormFieldOptionsActivityLogTest extends TestCase
         $payload = $event->properties->toArray();
         $this->assertArrayHasKey('options', $payload['old']);
         $this->assertArrayHasKey('options', $payload['new']);
-        // assertEquals: MySQL JSON columns may reorder associative-array
-        // keys on round-trip; structural equality is the contract.
-        $this->assertEquals(
-            [
+        // RFC-WS-6 session 2.7: activity log properties are canonicalized
+        // at write; assertSame on canonical encodings of both sides is
+        // byte-stable across MySQL JSON-column round-trip.
+        $this->assertSame(
+            JsonCanonicalizer::encode([
                 ['value' => 'a', 'label' => 'a', 'sort_order' => 0],
                 ['value' => 'b', 'label' => 'b', 'sort_order' => 1],
-            ],
-            $payload['old']['options'],
+            ]),
+            JsonCanonicalizer::encode($payload['old']['options']),
         );
-        $this->assertEquals(
-            [
+        $this->assertSame(
+            JsonCanonicalizer::encode([
                 ['value' => 'a', 'label' => 'A', 'sort_order' => 0],
                 ['value' => 'b', 'label' => 'b', 'sort_order' => 1],
                 ['value' => 'c', 'label' => 'c', 'sort_order' => 2],
-            ],
-            $payload['new']['options'],
+            ]),
+            JsonCanonicalizer::encode($payload['new']['options']),
         );
     }