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>

examples/component-showcase/app/pages/navigation.wrn

@@ -10,7 +10,7 @@ page NavigationShowcase {
       <span class="showcase-eyebrow">Component category</span>
       <h1 class="showcase-title">Navigation</h1>
       <p class="showcase-description">10 unique, responsive, theme-aware components. Open a component to inspect multiple live configurations and its complete props API.</p>
-      <div class="category-meta"><span>10 components</span><span>Multiple use cases</span><span>29 live configurations</span></div>
+      <div class="category-meta"><span>10 components</span><span>Multiple use cases</span><span>28 live configurations</span></div>
     </header>
     <div class="catalog-grid">
       <article class="catalog-card" data-interactive-preview="false">
@@ -76,7 +76,7 @@ page NavigationShowcase {
       <article class="catalog-card" data-interactive-preview="false">
         <div class="catalog-card-preview">
           <div class="catalog-card-glow" aria-hidden="true"></div>
-          <div class="catalog-card-stage"><Scrollspy size="default" label="Scrollspy" items='[{"id":"scrollspy-0-1","key":"scrollspy-0-1","value":"primary","label":"Primary workflow","title":"Primary workflow","description":"A clean default configuration for everyday product interfaces.","text":"A configurable sample message.","href":"#scrollspy-demo","actionHref":"#scrollspy-demo","actionLabel":"View details","icon":"icon-[lucide--sparkles]","iconClass":"icon-[lucide--sparkles]","variant":"primary","status":"Active","time":"09:30","current":true,"selected":true,"checked":true,"disabled":false,"outgoing":false,"open":true,"number":1,"count":16,"percentage":72,"color":"#7c3aed","target":"_self","ariaLabel":"Open primary workflow 1","items":[{"label":"Nested option A","value":"nested-a"},{"label":"Nested option B","value":"nested-b"}],"links":[{"label":"Documentation","href":"#documentation"},{"label":"API reference","href":"#api-reference"}],"values":["Included","Standard"]},{"id":"scrollspy-0-2","key":"scrollspy-0-2","value":"secondary","label":"Additional option","title":"Supporting example","description":"A clean default configuration for everyday product interfaces.","text":"Another configurable message.","href":"#scrollspy-demo","actionHref":"#scrollspy-demo","actionLabel":"View details","icon":"icon-[lucide--sparkles]","iconClass":"icon-[lucide--sparkles]","variant":"primary","status":"Active","time":"09:30","current":false,"selected":false,"checked":false,"disabled":false,"outgoing":true,"open":true,"number":2,"count":32,"percentage":72,"color":"#7c3aed","target":"_self","ariaLabel":"Open primary workflow 2","items":[{"label":"Nested option A","value":"nested-a"},{"label":"Nested option B","value":"nested-b"}],"links":[{"label":"Documentation","href":"#documentation"},{"label":"API reference","href":"#api-reference"}],"values":["Included","Standard"]}]' class="showcase-instance showcase-instance--1" /></div>
+          <div class="catalog-card-stage"><Scrollspy size="default" items='[{"label":"Overview","href":"#overview"},{"label":"Installation","href":"#installation"},{"label":"Usage","href":"#usage"},{"label":"API","href":"#api"}]' active="#overview" label="Scrollspy" heading="On this page" class="showcase-instance showcase-instance--1" /></div>
         </div>
         <div class="catalog-card-body">
           <div><span class="catalog-card-category">Navigation</span><h2>Scrollspy</h2><p>Theme-aware, responsive scrollspy component.</p></div>