feat(17-01): menu item type registry and dependent dropdown binding

- pact: HasMenuItemTypes, MenuItemType, MenuItemReference, MenuItemResolve,
  ResolvedMenuItem, MenuItemAlternate, DependentOptionsProvider,
  DependentOptionSet; Option gains Depth (omitempty)
- signpost (new module): boot-time registry with fail-loud key and pattern
  checks, layered URL patterns incl. PatternFrom for list types, host
  overrides at menu_items.patterns, Resolve and ReferenceOptions
- party: signpost.Activate after postcard, before Boot
- cabana: dependsOn on dropdowns (boot checks, controller-only), GET
  fields/{field}/dependent-options with swag, inventories, conformance case
- companions fixture, TestSignpostSmoke, TestCompanionsDependentSmoke, docs

modules/pact/README.md

@@ -30,6 +30,8 @@ Use cases:
 - 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`).
+- 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.
 - A background job contract (`pact.Job`, `pact.JobArgs`) that does not depend on any queue library.
 - A schedule contract: `pact.HasSchedule` returns `pact.ScheduledCommand` entries (a registered command name, its arguments and a `pact.Cadence` built with `pact.Daily`, `pact.DailyAt` or `pact.Every`), the Go form of WinterCMS `registerSchedule`. It does not depend on any queue library either.
 - `pact.OptionalMessage`, a service an optional plugin can publish so others integrate with it without importing its package.
@@ -156,7 +158,15 @@ func (p *Plugin) Schedule() []pact.ScheduledCommand {
 | `pact.RelationAfterUpdate` | Optional controller hook run after a relation manager updates a related record, before the commit. |
 | `pact.RelationBeforeDelete` | Optional controller hook run in the write transaction before a relation manager deletes a related record. |
 | `pact.RelationAfterDelete` | Optional controller hook run after a relation manager deletes a related record, before the commit. |
-| `pact.Option` | One dropdown choice (value and label). |
+| `pact.Option` | One dropdown choice (value and label), with `Depth`, the nesting level of a choice taken from a tree (omitted from JSON when 0). |
+| `pact.DependentOptionsProvider` | Optional controller hook serving the choices of a dropdown with `dependsOn` from the submitted values of its siblings; asked by the options route and on every save. |
+| `pact.DependentOptionSet` | `DependentOptions`' answer: `Applies` (false when the sibling's value takes no choice; the field then stores null) and `Options`. |
+| `pact.HasMenuItemTypes` | Implemented by plugins that register menu item types; collected once at boot by signpost. |
+| `pact.MenuItemType` | One menu item type: `Key`, `Label`, `DefaultPattern` (required, starts with `/`), `PatternFrom` (a list type's record type), `Placeholders`, `Nesting`, `DynamicItems`, the optional `References` func and the required `Resolve` func. |
+| `pact.MenuItemReference` | One record a menu item can point at: value, label and nested records. |
+| `pact.MenuItemResolve` | What `Resolve` receives: type, reference, the layered URL pattern, whether to expand nested items and the unprefixed locale. |
+| `pact.ResolvedMenuItem` | A resolved item: title, URL (empty for a pure list type), modification time, ordered alternates and nested items. |
+| `pact.MenuItemAlternate` | The URL and title of a resolved item in one locale. |
 
 ## Dependencies