(** The agent, as the compiler reaches it when both are in one process.

    [flan dev] builds one binary that is the compiled program and holds this
    compiler; the agent's listener thread and this one are threads in it. So a
    request that used to be a connect, a write and a read on a unix socket
    looping back into this address space is a call into
    [vendor/agent/flan_agent.c] instead.

    [None] means there is no agent in this process — the [flan] binary that
    builds the merged program, [flan reload], the two-process daemon, every
    test. The answer comes from the linker rather than from a flag: the symbol
    is weak in [dynload_stubs.c] and is null in a binary that links no
    [flan_agent.o]. [Dev] falls back to the socket on [None], which is what
    keeps [--two-process] working unchanged.

    What does *not* change is where a delivery lands. The agent still only
    publishes to its ring; the game thread picks the job up when it next
    reaches a frame boundary. The rule that keeps the collector away from the
    frame thread — the game thread never calls into OCaml, requests are left
    somewhere and picked up — is the same rule with the same mechanism. This is
    the compiler thread calling C, never the other way. *)

external request : string -> string option = "flan_agent_direct"