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>

dev-docs/ARCH-FORM-BUILDER.md

@@ -845,6 +845,28 @@ Rationale: using slugs (not IDs) in cross-references so snapshots are
 portable between orgs. library_field_id references are resolved to
 inline definitions at snapshot time (see §4.7).
 
+##### Byte-stability (RFC-WS-6 v1.1, session 2.7)
+
+The snapshot is audit-immutable. JSON content stored in
+`form_submissions.schema_snapshot` is canonicalized on write via
+`App\Support\Json\JsonCanonicalizer::canonicalize()` (recursive ksort
+on associative arrays; numeric-indexed lists preserve order). This
+guarantees that re-emits of the same logical content produce
+byte-identical JSON regardless of MySQL's JSON-column round-trip
+behavior. Critical for:
+
+- Audit-replay diffs (otherwise key-reorder shows up as false positives)
+- Webhook payload signing (HMAC requires byte-stable input;
+  `payload_snapshot` and the delivery-time `JsonCanonicalizer::encode`
+  re-encode produce the same bytes)
+- Activity log diff regression tests (`field.updated` `old`/`new`
+  payloads land via `FormField::logFieldChange` which canonicalizes
+  before `withProperties()`)
+
+Opaque-config columns (`form_schemas.settings`, `form_schemas.translations`)
+are NOT canonicalized — key order has no semantic meaning there. See
+`SchemaSnapshotByteStableAcrossReemitsTest` for the end-to-end contract.
+
 ### 4.7 `form_field_library` (cross-schema reusable definitions)
 
 | Column | Type | Notes |