Concurrent newline-delimited JSON-RPC transport.

A small coordinator owns framing and serializes complete stdout writes. Work is admitted through the optional, transport-neutral Snodo.Server.Executor, so the coordinator never awaits a handler and execution policy is reusable by other transports.

:write_timeout is a positive millisecond limit, defaulting to 5,000. A single linked, monitored helper performs each write while the coordinator waits boundedly; there is no writer queue. Cancellation and EOF handling can wait up to this limit during a blocked write. A timeout or output error is terminal: the transport cancels its work and closes subscriptions, without attempting further writes. A timed-out device may already have accepted some bytes, so the transport never retries. Supplied I/O devices are never stopped or killed.

:max_line_bytes limits one input message, defaulting to 2,000,000 bytes to match the HTTP listener's :max_body_bytes. A longer line is answered with -32600 and a null id without being decoded, and the next line is served normally. The VM's I/O server buffers each line in full before returning it, so this bounds decoding and dispatch, not that read buffer.