feat(18.1-01): task 3 flat reorder on the server

- config_reorder.yaml without parentColumn on a record with no parent_id
  column compiles as a flat (WinterCMS Sortable) reorder; maxDepth above 0
  on it fails boot, an explicit parentColumn still means a tree
- ListReorder.Flat and AdminReorderTree.Flat (flat, omitempty); flat nodes
  carry parent_id null, a save writes only the sort column, a parent is a
  422 on nodes (reorder.flat_parent); 409 and duplicate rules unchanged
- swag descriptions, OpenAPI and schema.d.ts regenerated
- acme.catalog badges reorder fixture, TestReorderFlatCompile and
  TestReorderFlatSmoke; docs Flat reorder section and cabana README

admin/openapi/admin.json

@@ -520,6 +520,10 @@
             },
             "cabana.AdminReorderTree": {
                 "properties": {
+                    "flat": {
+                        "description": "Flat is true for a flat reorder (WinterCMS Sortable): every node's\nparent_id is null and a save writes only the sort column. Omitted for\na tree reorder.",
+                        "type": "boolean"
+                    },
                     "nodes": {
                         "items": {
                             "$ref": "#/components/schemas/cabana.AdminReorderNode"
@@ -1945,6 +1949,10 @@
             },
             "cabana.ListReorder": {
                 "properties": {
+                    "flat": {
+                        "description": "Flat is true for a flat reorder (WinterCMS Sortable): the record has\nno parent column, so the records are only put in order, never nested.\nOmitted for a tree reorder.",
+                        "type": "boolean"
+                    },
                     "partition": {
                         "$ref": "#/components/schemas/cabana.ListReorderPartition"
                     },
@@ -4814,7 +4822,7 @@
         },
         "/{vendor}/{plugin}/{controller}/reorder": {
             "get": {
-                "description": "The records of a controller with config_reorder.yaml, scoped by the list scope (ListExtendQuery) and, when the page has a partition, by one partition: `partition` picks it (an id among `partition.options`; another id answers 404) and defaults to the first option. Nodes carry the id, parent_id (null at the top), sort_order and the nameFrom column as name, ordered by the sort column and then the id. A controller without a reorder page answers 404.",
+                "description": "The records of a controller with config_reorder.yaml, scoped by the list scope (ListExtendQuery) and, when the page has a partition, by one partition: `partition` picks it (an id among `partition.options`; another id answers 404) and defaults to the first option. Nodes carry the id, parent_id (null at the top), sort_order and the nameFrom column as name, ordered by the sort column and then the id. A flat reorder (no parentColumn key and no parent_id column on the record, WinterCMS Sortable) answers `flat: true` and parent_id null on every node; a tree reorder omits `flat`. A controller without a reorder page answers 404.",
                 "parameters": [
                     {
                         "description": "Vendor",
@@ -4915,7 +4923,7 @@
                 ]
             },
             "post": {
-                "description": "Saves the parent and sort columns of every record of the scoped set in one transaction that locks the set (ListExtendQuery plus the partition). `nodes` must list exactly the records of that set: any difference (a record added, removed or moved to another partition meanwhile) answers 409 `reorder_stale` and writes nothing. A duplicate id, a parent outside the set, a cycle and a depth above maxDepth answer 422 `validation_failed` on `nodes`; a missing partition on a partitioned page is a 422 on `partition`. The answer is the new tree.",
+                "description": "Saves the parent and sort columns of every record of the scoped set in one transaction that locks the set (ListExtendQuery plus the partition). `nodes` must list exactly the records of that set: any difference (a record added, removed or moved to another partition meanwhile) answers 409 `reorder_stale` and writes nothing. A duplicate id, a parent outside the set, a cycle and a depth above maxDepth answer 422 `validation_failed` on `nodes`; a missing partition on a partitioned page is a 422 on `partition`. On a flat reorder only the sort column is written, and a node with a non-null parent_id is a 422 on `nodes` (`backend::lang.reorder.flat_parent`). The answer is the new tree.",
                 "parameters": [
                     {
                         "description": "Vendor",