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/component-reference.json

@@ -10716,13 +10716,6 @@
       "category": "navigation",
       "purpose": "Theme-aware, responsive scrollspy component.",
       "props": [
-        {
-          "name": "size",
-          "type": "string",
-          "required": false,
-          "default": "\"default\"",
-          "options": []
-        },
         {
           "name": "color",
           "type": "string",
@@ -10731,10 +10724,10 @@
           "options": []
         },
         {
-          "name": "label",
+          "name": "size",
           "type": "string",
           "required": false,
-          "default": "\"Scrollspy\"",
+          "default": "\"default\"",
           "options": []
         },
         {
@@ -10752,10 +10745,17 @@
           "options": []
         },
         {
-          "name": "orientation",
+          "name": "label",
           "type": "string",
           "required": false,
-          "default": "\"horizontal\"",
+          "default": "\"On this page\"",
+          "options": []
+        },
+        {
+          "name": "heading",
+          "type": "string",
+          "required": false,
+          "default": "\"\"",
           "options": []
         },
         {
@@ -10770,7 +10770,7 @@
       "outputs": [
         {
           "name": "change",
-          "payloadType": "{ 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"
+          "payloadType": "{ href: string; label: string }"
         }
       ],
       "events": ["change"],