Pure logic layer for the top-right avatar role-switcher: derive the home
a principal is in and may switch into, mirroring nav.ts/auth.ts so the
decision is unit-tested without a React renderer.
- ViewMode is derived from NavSection["id"], so the three switchable
homes (User/Admin/SysAdmin) can never drift from the nav sections.
- availableViewModes / effectiveViewMode resolve a requested view against
the live is_admin flag, failing closed: a non-admin or a demoted admin
always collapses to the User home.
- restoreViewMode re-gates a persisted (localStorage) choice on every
read, never trusting the stored value over the live flag, closing the
one escalation vector a client-side persona could open.
- sectionsForView composes the view ceiling on top of visibleSections, so
the switcher only ever narrows the sidebar, never widens access.
The avatar dropdown UI that consumes this lands as a separate increment.
