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/cabana/testdata/companions/controllers/links/config_form.yaml

@@ -0,0 +1,4 @@
+name: New link
+form: ~/plugins/acme/companions/models/link/fields.yaml
+modelClass: Link
+defaultRedirect: acme/companions/links

modules/cabana/testdata/companions/controllers/links/config_list.yaml

@@ -0,0 +1,7 @@
+list: ~/plugins/acme/companions/models/link/columns.yaml
+modelClass: Link
+title: Links
+recordUrl: acme/companions/links/update/:id
+recordsPerPage: 20
+toolbar:
+    buttons: [create]

modules/cabana/testdata/companions/models/link/columns.yaml

@@ -0,0 +1,6 @@
+columns:
+    title:
+        label: Title
+        searchable: true
+    type:
+        label: Type

modules/cabana/testdata/companions/models/link/fields.yaml

@@ -0,0 +1,14 @@
+fields:
+    title:
+        label: Title
+        type: text
+        required: true
+    type:
+        label: Type
+        type: dropdown
+        options: typeOptions
+    reference:
+        label: Reference
+        type: dropdown
+        dependsOn: type
+        emptyOption: backend::lang.form.none