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/showcase-manifest.json

@@ -2,7 +2,7 @@
   "generatedFrom": "packages/ui/component-reference.json",
   "componentCount": 108,
   "categoryCount": 11,
-  "totalDemoCount": 420,
+  "totalDemoCount": 419,
   "components": [
     {
       "name": "Accordion",
@@ -990,11 +990,11 @@
       "slug": "scrollspy",
       "category": "navigation",
       "purpose": "Theme-aware, responsive scrollspy component.",
-      "demoCount": 3,
+      "demoCount": 2,
       "propCount": 7,
       "slots": ["default"],
       "events": ["change"],
-      "profiled": false
+      "profiled": true
     },
     {
       "name": "SearchBox",