feat(19.3-04): pact.ReorderAfterSave hook inside cabana's reorder transaction

- optional ReorderAfterSave(ctx, partition) runs after the new order is
  written; an error rolls the reorder back through lifecycleFailure
- reorderPost runs in lagoon.Transaction so lagoon.AfterCommit work a hook
  registers (a site-cache purge) runs only after the commit
- pact and cabana READMEs, admin hook table, tree reorder docs

modules/pact/README.md

@@ -29,7 +29,7 @@ Use cases:
 - Admin controller contracts: `pact.AdminController`, `pact.HasAdminControllers`, `pact.AdminAssets` (embedded Winter-shaped admin YAML), `pact.AdminPermissioned` and `pact.AdminRecordSource`.
 - Admin extension contracts, so a plugin extends the compiled admin SPA without a Node build: `pact.AdminClientAssets` (per-controller JS and CSS from the plugin's embedded `assets/` tree, Winter's `addJs`/`addCss`), `pact.HasAdminActions` with `pact.AdminAction`, `pact.AdminActionInput` and `pact.AdminActionResult` (named toolbar and widget actions whose routes, CSRF check, permissions and record scoping the framework owns), `pact.HasAdminBulkActions` with `pact.AdminBulkAction`, `pact.AdminBulkActionInput` and `pact.AdminBulkActionResult` (named actions on the rows selected in a list, which receive records the framework loaded through the list scope, never ids), `pact.HasAdminRecordActions` with `pact.AdminRecordAction`, `pact.AdminRecordActionInput` and `pact.AdminRecordActionResult` (named actions on one record, each with an `Applies` rule for the record's state), and `pact.AdminPartialData` (the curated view model a partial template renders).
 - Plugin field types: `pact.HasFormFieldTypes` returns `pact.FormFieldType` entries, admin form field types any controller's `fields.yaml` uses by a short alias (`type: notes`). A type names its custom element, its scripts and styles, its own option keys and its Go funcs: `Decode` turns a submitted `pact.FormFieldValue` into a `pact.FormFieldDecoded` (the value to store plus optional `pact.FormFieldNotice` messages for the administrator, such as what a sanitizer removed) or refuses it with a `pact.FormFieldError`; `Load` serves the stored value to the element, `ListValue` formats a list column, and `Upload` stores a file from a `pact.FormFieldUpload` and answers a `pact.FormFieldUploadResult`. The admin framework owns the routes, permissions, record scope, locale storage of a translatable type and the localization of notices.
-- Optional admin hooks a controller or model can implement: list and form query scoping (`pact.ListExtendQuery`, `pact.FormExtendQuery`), list row states (`pact.ListRowStates` with the fixed `pact.RowState` set `pact.RowStateDeleted`, `pact.RowStateNegative` and `pact.RowStateDisabled`), create, update and delete hooks (`pact.FormBeforeCreate`, `pact.FormAfterUpdate`, `pact.FormBeforeDelete` and their siblings), form-only fields that reach those hooks without being model columns (`pact.FormVirtualFields`), validation rules per operation for admin saves (`pact.FormRules`), relation hooks (`pact.RelationExtendManageQuery`, `pact.RelationExtendOptionsQuery`, `pact.RelationBeforeLink`), relation child hooks around creating, updating and deleting a related record (`pact.RelationBeforeCreate`, `pact.RelationAfterCreate`, `pact.RelationBeforeUpdate`, `pact.RelationAfterUpdate`, `pact.RelationBeforeDelete`, `pact.RelationAfterDelete`), filter scopes (`pact.FilterScope`, `pact.FilterOptions`) and dropdown options (`pact.DropdownOptionsProvider`).
+- Optional admin hooks a controller or model can implement: list and form query scoping (`pact.ListExtendQuery`, `pact.FormExtendQuery`), list row states (`pact.ListRowStates` with the fixed `pact.RowState` set `pact.RowStateDeleted`, `pact.RowStateNegative` and `pact.RowStateDisabled`), create, update and delete hooks (`pact.FormBeforeCreate`, `pact.FormAfterUpdate`, `pact.FormBeforeDelete` and their siblings), a hook after a reorder save (`pact.ReorderAfterSave`), form-only fields that reach those hooks without being model columns (`pact.FormVirtualFields`), validation rules per operation for admin saves (`pact.FormRules`), relation hooks (`pact.RelationExtendManageQuery`, `pact.RelationExtendOptionsQuery`, `pact.RelationBeforeLink`), relation child hooks around creating, updating and deleting a related record (`pact.RelationBeforeCreate`, `pact.RelationAfterCreate`, `pact.RelationBeforeUpdate`, `pact.RelationAfterUpdate`, `pact.RelationBeforeDelete`, `pact.RelationAfterDelete`), filter scopes (`pact.FilterScope`, `pact.FilterOptions`) and dropdown options (`pact.DropdownOptionsProvider`).
 - Dependent dropdown options: `pact.DependentOptionsProvider` answers a `pact.DependentOptionSet` (whether the sibling's value takes a choice at all, and the choices) for a dropdown with `dependsOn`, from the submitted values of its siblings and with a context, so the choices can come from the database; the admin framework calls it for the options route and again on every save. `pact.Option.Depth` indents a choice taken from a tree.
 - Menu item types: `pact.HasMenuItemTypes` returns `pact.MenuItemType` values (key, label, default URL pattern, `PatternFrom` for a list type, the `References` an item can point at as a `pact.MenuItemReference` tree, and `Resolve`), the typed successor of WinterCMS's `pages.menuitem.*` events. `Resolve` receives a `pact.MenuItemResolve` with the pattern already layered and answers a `pact.ResolvedMenuItem` with its alternates as an ordered `[]pact.MenuItemAlternate`. [signpost](../signpost/README.md) collects them at boot.
 - Singleton admin controllers: `pact.AdminSingleton` (`SingletonRecord`, first or create) makes a controller edit exactly one record, opened directly instead of from a list.
@@ -137,6 +137,7 @@ func (p *Plugin) Schedule() []pact.ScheduledCommand {
 | `pact.AdminRecordActionInput` | What a record action receives: `RecordID` and `Record`, the record loaded and row-locked through the form scope. |
 | `pact.AdminRecordActionResult` | What a record action returns: an optional message for the toast. |
 | `pact.HasAdminRecordActions` | Registers a controller's record actions for the `recordActions` list of `config_form.yaml`. |
+| `pact.ReorderAfterSave` | Optional controller hook `ReorderAfterSave(ctx, partition)`: runs after a reorder save wrote the new order, inside the same transaction (`cabana.TxFromContext` works); an error rolls the reorder back. |
 | `pact.AdminPartialData` | Supplies the view model a controller partial template renders; never the GORM model. |
 | `pact.HasFormFieldTypes` | Implemented by a plugin (not a controller) that registers admin form field types. |
 | `pact.FormFieldType` | One field type: the alias `Name`, the custom-element tag `Element`, `Translatable`, the `Scripts` and `Styles` under `assets/`, the `Options` keys, and the funcs `CheckOptions` (optional), `Decode` (required), `Load`, `ListValue` (`func(ctx context.Context, value any) string`) and `Upload` (optional). |

modules/pact/capabilities.go

@@ -765,6 +765,15 @@ type FormAfterDelete interface {
 	FormAfterDelete(ctx context.Context, model any) error
 }
 
+// ReorderAfterSave optionally runs after a reorder save (POST .../reorder of
+// a tree or sortable list) wrote the new order, inside the same transaction,
+// so cabana.TxFromContext returns it and lagoon.AfterCommit work waits for
+// the commit. partition is the reorder request's partition (nil when the
+// list has none). A returned error rolls the reorder back.
+type ReorderAfterSave interface {
+	ReorderAfterSave(ctx context.Context, partition *uint64) error
+}
+
 // FormVirtualFields optionally names fields of the controller's form that are
 // not model columns for the purpose of the form, such as a password and its
 // confirmation or a "send an invitation" checkbox. The admin framework never