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/Bindings/FormFieldBindingMigrationTest.php

@@ -6,6 +6,7 @@ namespace Tests\Feature\FormBuilder\Bindings;
 
 use App\Models\FormBuilder\FormSchema;
 use App\Models\Organisation;
+use App\Support\Json\JsonCanonicalizer;
 use Illuminate\Foundation\Testing\RefreshDatabaseState;
 use Illuminate\Support\Facades\Artisan;
 use Illuminate\Support\Facades\DB;
@@ -150,23 +151,30 @@ final class FormFieldBindingMigrationTest extends TestCase
         $this->artisan('migrate:rollback', ['--step' => 1])->assertSuccessful();
         $this->assertFalse(Schema::hasTable('form_field_bindings'));
 
-        // assertEquals: MySQL JSON columns may reorder associative-array
-        // keys on round-trip; structural equality is the contract here.
+        // RFC-WS-6 session 2.7: rollback writes JSON directly from
+        // migration code (not the canonicalizing service). Compare on
+        // canonical form so the assertion is engine-agnostic.
         $field = DB::table('form_fields')->where('id', $fieldAId)->first();
         $this->assertNotNull($field->binding);
-        $this->assertEquals([
-            'mode' => 'entity_owned',
-            'entity' => 'person',
-            'column' => 'email',
-        ], json_decode((string) $field->binding, true));
+        $this->assertSame(
+            JsonCanonicalizer::encode([
+                'mode' => 'entity_owned',
+                'entity' => 'person',
+                'column' => 'email',
+            ]),
+            JsonCanonicalizer::encode(json_decode((string) $field->binding, true)),
+        );
 
         $lib = DB::table('form_field_library')->where('id', $libAId)->first();
         $this->assertNotNull($lib->default_binding);
-        $this->assertEquals([
-            'mode' => 'entity_owned',
-            'entity' => 'person',
-            'column' => 'first_name',
-        ], json_decode((string) $lib->default_binding, true));
+        $this->assertSame(
+            JsonCanonicalizer::encode([
+                'mode' => 'entity_owned',
+                'entity' => 'person',
+                'column' => 'first_name',
+            ]),
+            JsonCanonicalizer::encode(json_decode((string) $lib->default_binding, true)),
+        );
     }
 
     /** @return array{0:string,1:string,2:string} */