Covers Kalshi's AsyncAPI 2.0.0 document (spec/asyncapi.yaml). Subscribe with WebSocketClient::subscribe(ws::Channel::..., params); each message arrives in on_message as ws::Update<T>.

Keep one OrderbookDelta handle per client. Use add_markets to extend it. A second subscribe returns InvalidRequest while an orderbook handle remains registered. Unsubscribing removes an unsent handle immediately; a rejected subscribe also removes its handle. Otherwise, wait for Unsubscribed or disconnect before replacing it. Updates are rejected while unsubscribe is pending; if the server rejects unsubscribe, retry the update on the same handle. Terminal channel errors 10 and 25 also remove the handle; subscribe again from on_error when ready to resume. Set update_interval_ms to V0, V100, or V1000, or omit it for zero milliseconds.

Channel	`ws::Channel`	Messages
`orderbook_delta`	`OrderbookDelta`	`ws::OrderbookSnapshot`, `ws::OrderbookDelta`
`ticker`	`Ticker`	`ws::Ticker`
`trade`	`Trade`	`ws::Trade`
`fill`	`Fill`	`ws::Fill`
`market_positions`	`MarketPositions`	`ws::MarketPosition`
`market_lifecycle_v2`	`MarketLifecycleV2`	`ws::EventLifecycle`, `ws::EventFeeUpdate`, `ws::MarketMetadataUpdated`, `ws::MarketLifecycleV2`
`multivariate_market_lifecycle`	`MultivariateMarketLifecycle`	`ws::EventLifecycle`, `ws::MultivariateMarketLifecycle`
`communications`	`Communications`	`ws::RfqCreated`, `ws::RfqDeleted`, `ws::QuoteCreated`, `ws::QuoteAccepted`, `ws::QuoteExecuted`, `ws::RfqBatchCreated`, `ws::RfqBatchDeleted`, `ws::BatchQuoteCreated`, `ws::RfqBatchBestQuotes`, `ws::BatchQuoteAccepted`, `ws::RfqBatchExecuted`, `ws::BatchQuoteExecuted`
`order_group_updates`	`OrderGroupUpdates`	`ws::OrderGroupUpdates`
`user_orders`	`UserOrders`	`ws::UserOrder`
`cfbenchmarks_value`	`CfbenchmarksValue`	`ws::CfbenchmarksValue`, `ws::CfbenchmarksIndexList`
`cfbenchmarks_value_5hz`	`CfbenchmarksValue5hz`	`ws::CfbenchmarksValue5Hz`, `ws::Cfbenchmarks5HzIndexList`
`pyth_value`	`PythValue`	`ws::PythValue`, `ws::PythUnderlyingList`