Write to a subprocess's stdin and/or close it

Writes a chunk to the stdin of a live process inside the running instance — spid=1 targets the main process, ≥2 the exec'd children. Requires the process to have been started with stdin.close=false; otherwise its pipe is already at EOF and the call returns 409.

close=true (the default) signals EOF after writing value; an empty value with close=true just closes the pipe. A client that serializes its own requests gets ordered writes; concurrent requests to the same spid are safe but their order is unspecified.

A 504 means the guest write timed out mid-call — the chunk may or may not have been delivered (e.g. the process stopped reading and the pipe is full), so blind retries can duplicate data.