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/http.go

@@ -438,7 +438,9 @@ func constrainFileID(g pact.Router) {
 func constrainNested(g pact.Router) {
 	constrainController(g)
 	g.Where("segment", "[A-Za-z_][A-Za-z0-9_]*")
-	g.Where("name", "[A-Za-z_][A-Za-z0-9_]*")
+	// name admits a hyphen for the literal dependent-options segment; a
+	// relation or file field name with one matches nothing and answers 404.
+	g.Where("name", "[A-Za-z_][A-Za-z0-9_-]*")
 }
 
 // nestedGet serves the logical routes
@@ -446,6 +448,7 @@ func constrainNested(g pact.Router) {
 //	GET /{vendor}/{plugin}/{controller}/{id}/relations/{name}
 //	GET /{vendor}/{plugin}/{controller}/{id}/files/{field}
 //	GET /{vendor}/{plugin}/{controller}/fields/{field}/options
+//	GET /{vendor}/{plugin}/{controller}/fields/{field}/dependent-options
 //	GET /{vendor}/{plugin}/{controller}/filters/{scope}/options
 //
 // A numeric id never equals a literal segment, so the dispatch is unambiguous.
@@ -456,6 +459,9 @@ func (s *service) nestedGet(w http.ResponseWriter, r *http.Request) {
 	case id == "fields" && name == "options":
 		r.SetPathValue("field", segment)
 		s.fieldOptions(w, r)
+	case id == "fields" && name == "dependent-options":
+		r.SetPathValue("field", segment)
+		s.dependentOptionsRoute(w, r)
 	case id == "filters" && name == "options":
 		r.SetPathValue("scope", segment)
 		s.filterOptions(w, r)