feat(ui): build Scrollspy, fix aria-current across the navigation group

Scrollspy replaces a scaffold that rendered bare anchors. The runtime observes
the sections the links point at and writes the marker straight onto the links:
an IntersectionObserver callback fires long after the client function that
registered it returned, so a state write there would be dropped.

Navbar and Breadcrumb both emitted aria-current="" for every inactive link.
That is not a valid value -- the attribute takes a token or must be absent --
so every link claimed a state it did not have. Breadcrumb had it too, despite
being the strongest component in the group.

Navbar also takes roving arrow-key focus across its menu bar.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

packages/ui/COMPONENTS.md

@@ -898,9 +898,9 @@ Theme-aware, responsive pagination component.
 Theme-aware, responsive scrollspy component.
 
 - Mount: `data-component="Scrollspy"`
-- Props: `size: string = "default"`, `color: string = "primary"`, `label: string = "Scrollspy"`, `items: unknown[] = []`, `active: string = ""`, `orientation: string = "horizontal"`, `class: string = ""`
+- Props: `color: string = "primary"`, `size: string = "default"`, `items: unknown[] = []`, `active: string = ""`, `label: string = "On this page"`, `heading: string = ""`, `class: string = ""`
 - Slots: `default`
-- Outputs: `change({ value?: string | number | boolean | null; values?: Array<string | number | boolean | null | object>; sourceEvent?: Event; [key: string]: string | number | boolean | null | object } | string | number | boolean | null)`
+- Outputs: `change({ href: string; label: string })`
 
 ### Sidebar