getEnsureExecutor

Public accessor for the package-private ENSURE_EXECUTOR so callers in other packages (e.g. RoutingMapManager) can post Carto-layer-mutating work on the same single thread that handles ensureNativeLayers. Required because Carto Mobile SDK 5.0.0 does not document Layers::add() as thread-safe — all native layer mutations must funnel through this one thread to avoid racing the GL render loop.