Subscribe to notification channels

WebSocket-only method that subscribes the connection to one or more notification channels by name — see the Subscriptions section for the available channels and their address formats (e.g. orderbook.ETH-PERP.10.20). Duplicate names are deduplicated; private channels require a logged-in connection with access to the referenced subaccount. Returns a per-channel status map (“ok” or an error string) plus the connection’s full set of subscriptions after the call.