Pure data-shape logic moves out of the client components so page.tsx can
compute the section list on the server without importing them.
The secondary page is not a variant of the primary one -- different section
ids (gcse, wellbeing), different flags, and History gated on more than one
year rather than at least one -- so it gets its own computeSecondaryFlags and
buildSecondaryNavItems rather than bending a shared function.
Adds 16 unit tests covering the all-through and special-school branches,
previously reachable only through a full component render.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
