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

modules/cabana/README.md

@@ -41,6 +41,7 @@ Use cases:
 - Rich text: `type: richeditor` edits HTML in a host text column (the admin SPA shows a WYSIWYG editor). It takes the common field keys plus `size`; any other key fails boot, and settings and relation forms refuse the type (supported in controller forms only). On create and update a submitted value must be a string or null (anything else is a 422 on the field) and is sanitized through [seaglass](../seaglass/README.md) before it is filled: HTML the policy keeps whole is stored byte for byte, otherwise the clean HTML is stored and what was removed is reported in `meta.notices` under the field (`backend::lang.richeditor.removed_elements`, `removed_attributes`, `removed_comments` with the count and names, or `markup_changed`).
 - Repeater: `type: repeater` edits a JSON list in one column (`lagoon.Jsonable`) with a nested `form.fields` of `text`, `textarea`, `number`, `checkbox`, `switch`, `dropdown` (static, method or `dependsOn` a field of the same row) and `richeditor` fields, plus `prompt`, `minItems`, `maxItems` and `titleFrom` (`cabana.RepeaterForm` in the schema; the span is always full). Other nested types, a nested repeater, nested `tab`, `context` or `preset`, and WinterCMS's `groups` and `style` fail boot; settings and relation forms refuse the type. A save decodes the rows before the projection: null is an empty list, anything but a list of objects is a 422 on the field, each row keeps only the nested names with scalars coerced per type, a required nested value missing in a row is a 422 under `field.index.nested`, the row bounds are 422s on the field (`repeater.max_items`, `repeater.min_items`), nested richeditor values are sanitized with notices under the row path, and nested dependent dropdowns are rechecked. Record responses carry the list of rows.
 - Tree reorder (D-09): a `config_reorder.yaml` (`title`, `nameFrom`, `modelClass`, `parentColumn` default `parent_id`, `sortColumn` default `sort_order`, `maxDepth`, and an optional `partition` with `column`, `relation`, `nameFrom` and `label`) gives the controller a reorder page and the built-in `reorder` toolbar button; unknown keys, columns that are not record columns, a partition relation that is not a belongsTo relation field over the column, and a `reorder` button without the file fail boot. The list schema carries `reorder` (`cabana.ListReorder`). GET `.../reorder` answers the scoped tree (`cabana.AdminReorderTree`); POST `.../reorder` locks the scoped set (ListExtendQuery plus the partition) in one transaction, answers 409 `reorder_stale` when the posted ids differ from it in any way, 422 for a duplicate, a foreign parent, a cycle or a depth above `maxDepth`, and otherwise writes both tree columns without model hooks.
+- Flat reorder (D-17): a `config_reorder.yaml` without `parentColumn` on a record that has no `parent_id` column (WinterCMS Sortable) compiles as a flat reorder: the list schema's `reorder` and the reorder routes' data carry `flat: true` (omitted on a tree, so tree payloads are unchanged), every node's `parent_id` is null, a save writes only the sort column, a node with a parent is a 422 on `nodes` (`backend::lang.reorder.flat_parent`), and the 409 stale-set and duplicate-id rules are unchanged. `maxDepth` above 0 on a flat reorder fails boot; an explicit `parentColumn` still means a tree.
 - Singleton controllers (D-11): a controller implementing `pact.AdminSingleton` needs only `config_form.yaml`; GET `.../singleton` answers the id of the record `SingletonRecord` returns (created when absent; a first request that loses the creation race on a unique index runs once more and finds the record) after checking the form scope, the form schema carries `singleton`, and the list, create, bulk and delete routes answer 404. A settings link entry may open it.
 - Dependent dropdowns: `dependsOn: <field>` on a `type: dropdown` field names a sibling with a scalar value (the same form, or the same repeater row); the dropdown takes no `options` and its controller implements `pact.DependentOptionsProvider`, else boot fails. GET `.../fields/{field}/dependent-options?values[<sibling>]=<value>` answers `cabana.AdminDependentOptionsResult` (`applies` and the localized `options` with their `depth`); `{field}` is `repeater.field` for a dropdown in a repeater row; the form schema carries `dependsOn`. Every save rechecks the value with the submitted sibling value (per row in a repeater): a value that does not apply is stored null, one outside the options is a 422 `form.dependent_invalid` on the field. A menu or sitemap controller returns `signpost.ReferenceOptions` here, the one admin binding of the [signpost](../signpost/README.md) menu item type registry. Settings and relation forms refuse `dependsOn` (supported in controller forms only).
 - Preset fields: `preset` on a `type: text` or `type: mltext` field (a source field name, or a mapping with `field` and `type`, `slug` or `exact`) makes the field follow another text or mltext field of the same form on the create screen until the administrator edits it, per locale for an mltext pair. The schema reports it as `preset` (`cabana.FieldPreset`); the server does not fill the field.
@@ -247,8 +248,8 @@ func (p *Plugin) AdminFS() fs.FS { return adminFS }
 | `cabana.AdminBulkAction` | Swag annotation of the bulk action route. |
 | `cabana.AdminDependentOptionsResult` / `cabana.AdminDependentOption` | Answer of the dependent-options route: `applies` and the options, each with `value`, localized `label` and `depth`. |
 | `cabana.AdminDependentOptions` | Swag annotation of the dependent-options route. |
-| `cabana.ListReorder` / `cabana.ListReorderPartition` | The list schema's `reorder` object: the page title and the partition column and label. |
-| `cabana.AdminReorderTree` / `cabana.AdminReorderNode` / `cabana.AdminReorderPartition` / `cabana.AdminReorderPartitionOption` | Answer of the reorder routes: title, partition (label, options, value) and the nodes with `id`, `parent_id`, `sort_order` and `name`. |
+| `cabana.ListReorder` / `cabana.ListReorderPartition` | The list schema's `reorder` object: the page title, the partition column and label, and `Flat` (`flat`, omitted when false) for a flat reorder. |
+| `cabana.AdminReorderTree` / `cabana.AdminReorderNode` / `cabana.AdminReorderPartition` / `cabana.AdminReorderPartitionOption` | Answer of the reorder routes: title, partition (label, options, value), the nodes with `id`, `parent_id`, `sort_order` and `name`, and `flat` (omitted when false) on a flat reorder. |
 | `cabana.AdminReorderRequest` / `cabana.AdminReorderNodeInput` | Body of the reorder save. |
 | `cabana.AdminReorderGet` / `cabana.AdminReorderSave` | Swag annotations of the reorder routes. |
 | `cabana.AdminSingletonResult` / `cabana.AdminSingletonRecord` | Answer and swag annotation of the singleton route. |