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/relation_form.go

@@ -159,6 +159,9 @@ func compileRelationForm(pluginID string, ctl pact.AdminController, fsys fs.FS,
 		if field.Preset != nil {
 			return nil, fail(fmt.Errorf("field %s: preset is not supported in a relation form (%s)", field.Name, purpose))
 		}
+		if err := controllerFormOnly(field); err != nil {
+			return nil, fail(fmt.Errorf("field %s: %w", field.Name, err))
+		}
 		if cr.hasMany() && field.Name == cr.Contract.ForeignKey {
 			return nil, fail(fmt.Errorf("field %s is the relation's ForeignKey; the server sets it", field.Name))
 		}