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/scripts/showcase-profiles.mjs

@@ -253,6 +253,38 @@ const DATATABLE_ROWS =
   '[{"id": 1, "name": "Northwind", "plan": "Scale", "owner": "A. Okafor", "seats": 6, "status": "Active", "statusHtml": "<span class=\\"showcase-pill showcase-pill--success\\">Active</span>"}, {"id": 2, "name": "Acme Industrial", "plan": "Team", "owner": "R. Silva", "seats": 13, "status": "Trial", "statusHtml": "<span class=\\"showcase-pill showcase-pill--info\\">Trial</span>"}, {"id": 3, "name": "Globex", "plan": "Enterprise", "owner": "M. Chen", "seats": 20, "status": "Past due", "statusHtml": "<span class=\\"showcase-pill showcase-pill--danger\\">Past due</span>"}, {"id": 4, "name": "Initech", "plan": "Starter", "owner": "J. Dubois", "seats": 27, "status": "Active", "statusHtml": "<span class=\\"showcase-pill showcase-pill--success\\">Active</span>"}, {"id": 5, "name": "Umbrella", "plan": "Scale", "owner": "P. Novak", "seats": 34, "status": "Trial", "statusHtml": "<span class=\\"showcase-pill showcase-pill--info\\">Trial</span>"}, {"id": 6, "name": "Stark Labs", "plan": "Team", "owner": "A. Okafor", "seats": 41, "status": "Past due", "statusHtml": "<span class=\\"showcase-pill showcase-pill--danger\\">Past due</span>"}, {"id": 7, "name": "Wayne Foods", "plan": "Enterprise", "owner": "R. Silva", "seats": 48, "status": "Active", "statusHtml": "<span class=\\"showcase-pill showcase-pill--success\\">Active</span>"}, {"id": 8, "name": "Soylent", "plan": "Starter", "owner": "M. Chen", "seats": 55, "status": "Trial", "statusHtml": "<span class=\\"showcase-pill showcase-pill--info\\">Trial</span>"}, {"id": 9, "name": "Hooli", "plan": "Scale", "owner": "J. Dubois", "seats": 62, "status": "Past due", "statusHtml": "<span class=\\"showcase-pill showcase-pill--danger\\">Past due</span>"}, {"id": 10, "name": "Vehement", "plan": "Team", "owner": "P. Novak", "seats": 69, "status": "Active", "statusHtml": "<span class=\\"showcase-pill showcase-pill--success\\">Active</span>"}, {"id": 11, "name": "Massive Dynamic", "plan": "Enterprise", "owner": "A. Okafor", "seats": 76, "status": "Trial", "statusHtml": "<span class=\\"showcase-pill showcase-pill--info\\">Trial</span>"}, {"id": 12, "name": "Cyberdyne", "plan": "Starter", "owner": "R. Silva", "seats": 83, "status": "Past due", "statusHtml": "<span class=\\"showcase-pill showcase-pill--danger\\">Past due</span>"}, {"id": 13, "name": "Tyrell", "plan": "Scale", "owner": "M. Chen", "seats": 90, "status": "Active", "statusHtml": "<span class=\\"showcase-pill showcase-pill--success\\">Active</span>"}, {"id": 14, "name": "Aperture", "plan": "Team", "owner": "J. Dubois", "seats": 97, "status": "Trial", "statusHtml": "<span class=\\"showcase-pill showcase-pill--info\\">Trial</span>"}, {"id": 15, "name": "Black Mesa", "plan": "Enterprise", "owner": "P. Novak", "seats": 104, "status": "Past due", "statusHtml": "<span class=\\"showcase-pill showcase-pill--danger\\">Past due</span>"}]';
 
 export const componentProfiles = {
+  Scrollspy: {
+    demos: [
+      standard(
+        "Table of contents",
+        "Each href points at an element on the page. The runtime observes those elements and moves aria-current to the link for whichever one is in view, so the marker follows the reader without any component state.",
+        {
+          heading: "On this page",
+          items: [
+            { label: "Overview", href: "#overview" },
+            { label: "Installation", href: "#installation" },
+            { label: "Usage", href: "#usage" },
+            { label: "API", href: "#api" },
+          ],
+          active: "#overview",
+        },
+      ),
+      advanced(
+        "Compact rail",
+        "On a phone the rail becomes a horizontal strip that scrolls, so a long contents list costs one line rather than a screenful.",
+        {
+          heading: "Sections",
+          size: "sm",
+          items: [
+            { label: "Getting started", href: "#getting-started" },
+            { label: "Configuration", href: "#configuration" },
+            { label: "Troubleshooting", href: "#troubleshooting" },
+          ],
+          active: "#configuration",
+        },
+      ),
+    ],
+  },
   MegaMenu: {
     demos: [
       standard(