Breadcrumb

The path to the current page — only when the hierarchy is real and more than two levels deep.

Segments derive from the route table. Every segment before the last navigates to its own route — href="#" is a bug.
The last segment is BreadcrumbPage and is never a link.
A namespace parent with no screen renders as plain text, not a dead link.
Breadcrumbs live in exactly one place in the shell — the page header, never also the top bar.