v2.1.0 - Host integration seams

Four additive seams for host platforms embedding the subscriptions engine. SubscriptionService::currentForTenants() gives platform-authority callers a bounded, single-query bulk projection of tenant subscriptions. OverrideRepository::listForSubject() exposes full override rows including expiry and reason metadata for administrative UIs. SubscriptionSubjectDataPurger::countSubjectRows() provides non-mutating per-table counts that share the purge predicates exactly, for purge prepare/verify flows. SubscriptionSchemaReadiness::isReady() is the extension-owned schema readiness authority — a never-fatal probe of the complete 2.x shape for host degraded modes. No behavior changes for existing callers. Also amends the v2 spec's resolver wording: binding a host resolver enables memberships only when it positively resolves and validates user subjects.