REVIEW-001 (M0) L7c: shouldNudge trusted the request Host, so a reverse
proxy that rewrites Host wrongly suppressed (or enabled) the nudge.
Prefer the configured server.base_url host (Deps.BaseURL) and fall back to
the request Host only when it is unset. Wire the secure session-cookie name
through SessionCookieNameFor so the `__Host-` prefix (L1s) is used
consistently by the flows.
- webauthn: the passkey ceremony service (a fake in tests)
- totp: the TOTP authenticator; the secret is AEAD-sealed at rest (A9)
- sealer: the AEAD TOTP sealer
- identities/sessions/credentials: the GUID state machine
- flow: the enroll/login screens + the passkey nudge (pre-session
double-submit CSRF — A15), rendered inside the shell
