Extracts the default-loadout body into a private BuildDefaultViewer
helper shared by the existing Steam-import path and a new
RegisterAnonymousViewer for /tool/signup. LinkSteamToViewer is the
seam SteamSessionAuthenticationHandler will call on first-Steam-touch
of a UDID-keyed viewer.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>