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

README.md

@@ -109,6 +109,7 @@ The `migrate`, `migrate:status`, `migrate:rollback`, `serve` and admin commands
 | [phrasebook](modules/phrasebook/README.md) | Namespaced translation catalogs loaded from plugin YAML, with locale fallback, placeholder interpolation and CLDR pluralization. | What you pack to speak the local language on holiday; it holds the translations. |
 | [postcard](modules/postcard/README.md) | Transactional mail from plugin-owned Markdown templates and layouts, delivered through a memory, log or SMTP driver. | What you send home in summer; it delivers the application's mail. |
 | [sieve](modules/sieve/README.md) | Generic, dependency-free collection helpers that the standard library slices and maps packages lack: mapping, filtering, reducing, keying, grouping, deduplicating, partitioning and finding, plus a small set type. | A sand sieve: you shake a collection through it and keep what you need; map, filter and reduce for slices and maps. |
+| [signpost](modules/signpost/README.md) | Menu item type registry that collects the typed menu item types plugins register at boot, layers their URL patterns with host overrides and resolves menu and sitemap items through the owning plugin. | A beach signpost points the way to places; the registry tells menus and sitemaps where each item leads. |
 | [sunrise](modules/sunrise/README.md) | Bootstrap script that installs the summer CLI and creates a new SummerCMS application, or updates an existing one, in one command, embedded so it is versioned with the framework. | The start of a summer day; it starts a new application, or updates one, in one command. |
 | [sunscreen](modules/sunscreen/README.md) | Credential-redacting slog handler that keeps API keys, tokens and passwords out of application logs. | Protects you from exposure; it keeps secrets out of the logs. |
 | [surf](modules/surf/README.md) | HTTP routing for SummerCMS: collects plugin routes and named middleware into a `net/http` ServeMux with constraints, rate limiting, body limits, CORS and panic recovery, and provides the `serve` and `route:list` commands. | Catching the incoming waves; it routes incoming HTTP requests to their handlers. |