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>

docs/ui-visual-contract-0.8.json

@@ -13,7 +13,7 @@
     "packages/ui/components/BackToTop.wrn": "b12c56ec8b7aabad68cbdc47f4b3237e2c21a61398f0aa51f9add6229b7357cd",
     "packages/ui/components/Badge.wrn": "6b55b4d85100d605cf168857aec1dc57da62c1f6a0a5cd6dd877023f24b189cc",
     "packages/ui/components/Blockquote.wrn": "6016dd4450de7cbf5c3cb413599baa2e502321e8eedef54439a0df6d0aae8bcb",
-    "packages/ui/components/Breadcrumb.wrn": "8bf6239b95b382ce68f8411aac0d364f67a1c278dfbbf08f1501cd0482751e30",
+    "packages/ui/components/Breadcrumb.wrn": "df27101d41cf018d55b6909e0399286a4661637140e341c8624615051d485142",
     "packages/ui/components/ButtonGroup.wrn": "0d97fbeed531d1d8ef4b59531923b47b34100d0de71d7bf524b02c668920b3d8",
     "packages/ui/components/CTASection.wrn": "2d2241d4f1018cfd2f8ea1ddd5fa8c8fcb8d1bbb2d4e4498cb9f39ff47a3801c",
     "packages/ui/components/Card.wrn": "4370df341235819b8a968a3f812209c2e6c29e4471d06578aaa5b577fb6e2dc8",
@@ -66,7 +66,7 @@
     "packages/ui/components/MetricGrid.wrn": "6018a98c10628ed240ed236ed916c0ff60994d192c3aadafd10bc876be0f9364",
     "packages/ui/components/Modal.wrn": "59d4ae9d6dd2700692d9edecfe53ee868e9f864363a4885961d1813cb2bee51f",
     "packages/ui/components/Nav.wrn": "7dc456b879e9248bde267b986cdcb138b8f3127d98b51e27bc9708129953f5e2",
-    "packages/ui/components/Navbar.wrn": "e68f9d3643e500e43124e9c7a6d4c7f6722657377ea7313f3e3cf5093a81b360",
+    "packages/ui/components/Navbar.wrn": "cf28cf4cdd23c85821d3302e6881115c2c14977e7cb7142d306cf559fea2a518",
     "packages/ui/components/PageHeader.wrn": "0761235a4924eec09877be40b292d27b70db22d210be7d8e989493165c20df86",
     "packages/ui/components/Pagination.wrn": "9ceb74745b159150b015bbbb1974c68e14a7d4b92bd03491cb23b8855aa07983",
     "packages/ui/components/PinInput.wrn": "5196f584de8d548a5dfa03688c3c95da3c948cead2662b05e927386ccea74299",
@@ -77,7 +77,7 @@
     "packages/ui/components/Radio.wrn": "0b2d38a5aee3e859280e4590fa1d509d789cc2239082054872fa342817735f1b",
     "packages/ui/components/RangeSlider.wrn": "21eb7a5df0ee2cb5e78f628eb920265998eb9f1a4933d4e5c57db0323fd66b41",
     "packages/ui/components/Rating.wrn": "a2d74dc748fc7d98892684c760518b87fa65411b1102e3611252b87245b3ffcb",
-    "packages/ui/components/Scrollspy.wrn": "bbe0788f63c2bc8850649c1dee78391d485a1c93876015bc536d838273d5fae4",
+    "packages/ui/components/Scrollspy.wrn": "76d1c17580c8d460f1222ce97585bdcac9779141f2f30d5460c84ff9f75413d8",
     "packages/ui/components/SearchBox.wrn": "315f54f1feaaa47a815d1e2d7a35b492f09fbfadfc6d37228e1009e19e0652ad",
     "packages/ui/components/Section.wrn": "21485de07d8bb986837a65c61c5dd01ba6c2f3d4e37d58c154a584c7b58de0ed",
     "packages/ui/components/SectionHeader.wrn": "512cb96636ee1f0540a0f4b4c75603fc0fe7536e7ed7ed7dc82173a16f48a4d3",