d78dd6d turned /sirius/ into a redirect to /sirius-x/, so the Sirius nav tab
left the shared header and dropped visitors into the sovereign Sirius.X site.
Per user: /sirius/ is the subdirectory page with the common header; Sirius.X
stays the sovereign registrar. Restored the page from 944080d and pointed
every register CTA at /sirius-x/#search-input, with a note listing the
mirror, gateway and native sirius.x URLs. register.html keeps its redirect.