a607cea110
Phase 3 validated a stored ID token hint against the currently configured issuer, but the oversize path never got that check: an ID token above the 3.9KB cookie limit was stored as the bare string 'sso', which collapsed to an undefined hint at logout and skipped validation entirely. Changing the issuer while such a session was live bounced the user to the new IdP on logout. Stores sso.<base64url(issuer)> instead and moves all marker interpretation into buildEndSessionUrl: raw ID token -> iss/aud-validated hint, issuer-tagged marker -> round-trip without a hint, anything else -> no round-trip. Every branch fails closed. Refs #798.