A signup-allowlisted partner owner (brand_signup_allowlist) could sign up
on an invite-only brand domain but was then exiled to the canonical domain
by the middleware home-domain affinity rule before any team membership
existed: the layout-level exemption in resolveBrandDomainBounce was
unreachable because the middleware redirect runs first. Rule 2 now checks
the allowlist (fail-closed) and lets those sessions stay, caching the
verdict like other home verdicts. Found via the Ziffr owner lockout.
Claude-Session: https://claude.ai/code/session_011eQDGWLppyy6BZ6Rx6shjd
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>