Add incremental output support and simplify RPC layer
Replace CBOR/channel-based transport with direct message passing.
Add OutputAt message type for incremental cell output streaming.
Remove unused channel.ml/mli and rpc_cbor.ml/mli modules.
Update client libraries with worker blob URL creation helper.
Add node-based incremental output test.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>