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