Phase 0, Task 0.2. Single source of truth for 'is the auth gate active?'.
Reuses the existing _LOOPBACK_HOST_VALUES frozenset so this stays in sync
with the DNS-rebinding host-header check. RFC1918/CGNAT/link-local are
treated as public — exact threat model the gate exists for.
Phase 0, Task 0.1 of the dashboard-oauth plan. Establishes a baseline for
the loopback dashboard's auth surface so future phases can prove they
didn't regress the existing _SESSION_TOKEN flow when adding the OAuth gate.
Ben