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/security_coverage_test.go

@@ -58,6 +58,7 @@ var phase09Routes = []adminRoute{
 	{key: "GET /{vendor}/{plugin}/{controller}/schema/relation/{name}"},
 	{key: "GET /{vendor}/{plugin}/{controller}/partials/{name}"},
 	{key: "GET /{vendor}/{plugin}/{controller}/fields/{field}/options", mounted: nestedGetRoute},
+	{key: "GET /{vendor}/{plugin}/{controller}/fields/{field}/dependent-options", mounted: nestedGetRoute},
 	{key: "GET /{vendor}/{plugin}/{controller}/filters/{scope}/options", mounted: nestedGetRoute},
 	{key: "GET /{vendor}/{plugin}/{controller}"},
 	{key: "POST /{vendor}/{plugin}/{controller}"},
@@ -316,6 +317,7 @@ func phase09ProtectedCalls() []phase09Call {
 		{"relation-schema", (*service).relationSchema},
 		{"relation-linked", (*service).relationLinked},
 		{"field-options", (*service).fieldOptions},
+		{"dependent-options", (*service).dependentOptionsRoute},
 		{"filter-options", (*service).filterOptions},
 		{"nested-get", (*service).nestedGet},
 		{"relation-candidates", (*service).relationCandidates},