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/app/Services/FormBuilder/FormSubmissionService.php

@@ -19,9 +19,9 @@ use App\Models\FormBuilder\FormSubmission;
 use App\Models\FormBuilder\FormSubmissionDelegation;
 use App\Models\FormBuilder\FormValue;
 use App\Models\User;
+use App\Support\Json\JsonCanonicalizer;
 use Illuminate\Database\Eloquent\Model;
 use Illuminate\Support\Facades\DB;
-use Illuminate\Support\Str;
 
 /**
  * Submission lifecycle: draft → submitted → reviewed per ARCH §4.3, §15.
@@ -103,7 +103,7 @@ final class FormSubmissionService
     {
         $this->assertWritable($submission);
 
-        $result = DB::transaction(function () use ($submission, $actor): FormSubmission {
+        $result = DB::transaction(function () use ($submission): FormSubmission {
             $schema = $submission->schema;
 
             $submission->status = FormSubmissionStatus::SUBMITTED->value;
@@ -111,7 +111,12 @@ final class FormSubmissionService
             $submission->schema_version_at_submit = $schema->version;
 
             if ($schema->snapshot_mode !== FormSchemaSnapshotMode::NEVER) {
-                $submission->schema_snapshot = $this->buildSnapshot($schema);
+                // RFC-WS-6 session 2.7: schema_snapshot is audit-immutable;
+                // canonicalize before storage so MySQL JSON-column round-trip
+                // can never corrupt audit-replay diffs or webhook signing.
+                $submission->schema_snapshot = JsonCanonicalizer::canonicalize(
+                    $this->buildSnapshot($schema),
+                );
             }
 
             if ($submission->opened_at !== null) {
@@ -272,7 +277,6 @@ final class FormSubmissionService
      * any residual options key defensively (commit 2 backfill should
      * already have done so on existing rows).
      *
-     * @param  mixed  $translations
      * @return array<string, mixed>|null
      */
     private function stripOptionsFromTranslations(mixed $translations): ?array

api/app/Services/FormBuilder/FormWebhookDispatcher.php

@@ -9,6 +9,7 @@ use App\Jobs\FormBuilder\DeliverFormWebhookJob;
 use App\Models\FormBuilder\FormSchemaWebhook;
 use App\Models\FormBuilder\FormSubmission;
 use App\Models\FormBuilder\FormWebhookDelivery;
+use App\Support\Json\JsonCanonicalizer;
 
 /**
  * Finds active webhooks for a submission's schema + trigger and queues a
@@ -36,7 +37,12 @@ final class FormWebhookDispatcher
                 'trigger_event' => $triggerEvent,
                 'status' => FormWebhookDeliveryStatus::PENDING->value,
                 'attempts' => 0,
-                'payload_snapshot' => $this->buildPayload($submission, $triggerEvent),
+                // RFC-WS-6 session 2.7 — canonicalize before storage; the
+                // delivery job HMAC-signs the same canonical bytes after
+                // re-encode, so signature is reproducible.
+                'payload_snapshot' => JsonCanonicalizer::canonicalize(
+                    $this->buildPayload($submission, $triggerEvent),
+                ),
             ]);
 
             DeliverFormWebhookJob::dispatch($delivery->id)->onConnection('webhooks')->onQueue('webhooks');